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Caramelized Onion and Arugula Tostadas

Onions that practically melt in your mouth, peppery arugula, fruity acidic tomato and creamy avocado, all assembled on a crispy tortilla, maybe topped with white cheddar cheese—these caramelized onion and arugula tostadas.

Caramelized Onion and Arugula Tostadas on baking sheet with lime wedges

Tostadas are basically open face tacos, with the filling on top instead of inside.  Even though they are not what you think of when you think Mexican food, caramelized sweet onions and spicy arugula give a tasty twist to tostadas. Finish with grated cheese and a drizzle of tangy yogurt if you wish. With creamy avocado and the tender caramelized onions, these tostadas are delicious even made vegan, without the dairy.

Click here to PIN Caramelized Onion and Arugula Tostadas.
closeup of Caramelized Onion and Arugula Tostadas on baking sheet


The great thing about tostadas is they are build-your-owns. Toast the tortillas so they are ready and waiting. Set out bowls of the topping ingredients– the onions, shredded arugula, tomatoes, avocados, cilantro, salsa, grated cheese, yogurt and lime wedges.  At the table people can layer their choices, adding more of what they want, skipping what they don’t.

You can buy diminutive (4-inch) round corn tortillas called “taco sliders.” Toasted slider tortillas are easier to manage in your hands (and mouth) than the 6-inch tortillas. Plus you can eat several and feel good about it. Robbie gobbled up six–these tostadas are much too tempting. Mi Rancho taco sliders are my favorites.

Tostadas come with their own crispy edible plate–and are fun for dinner or as casual snacks.

Caramelized Onion and Arugula Tostadas

More than tostadas, Mexico has been on my mind lately. A week ago, at the southern tip of Baja, just before their Independence Day holiday celebration, the flag blew off the flagpole. Hurricane Odile took center stage. Making these caramelized onion and arugula tostadas, my thoughts kept returning to Baja Mexico.

onions in skillet for Caramelized Onion and Arugula Tostadas

Caramelized Onion and Arugula Tostadas recipe details:

  • Look for Walla-Walla, Vidalias, or other “sweet” onion varietals. Sweet onions caramelize well because of their higher percentage of natural sugar.
  • I used the medium slicing blade (4 mm) that came with my food processor to slice the onions. No tears.
  • Cook the onions slowly. If they start to brown too quickly, turn down the heat; don’t hurry their caramelization. It seems like a lot of onions at the start, but by the time they’ve caramelized you will wonder if you have enough to top your tostadas.
  • Make them vegan: Skip the cheese and drizzle with guacamole avocado sauce.

caramelized onions in skillet for Caramelized Onion and Arugula Tostadas

More meatless tostada ideas from other bloggers:

Wishing you a fabulous week–get in the kitchen and cook something healthy, spicy and delicious!

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5 from 2 votes
Caramelized Onion and Arugula Tostadas
Caramelized Onion and Arugula Tostadas
Prep Time
15 mins
Cook Time
1 hr
Total Time
1 hr 15 mins

Serve these corn tortilla treats as snacks, or with a green salad for a simple supper.  Vegans--skip the cheese and yogurt garnish. Makes 4 (6-inch) or 10 smaller (4-inch) tostadas.

Course: Appetizer, Main Course
Cuisine: Mexican, Vegetarian
Keyword: vegetarian, tostadas, caramelized onions
Servings: 4 6-inch tacos or 10 (4-inch) tostadas.
  • 2 large or 3 smaller sweet onions, thinly sliced (see note)
  • 2 tablespoons olive oil
  • sea salt
  • 4 or 10 4 (6-inch) or 10 (4-inch) corn tortillas
  • cooking spray, as needed
  • 1 ½ cups shredded arugula (see note)
  • salsa, homemade or purchased, as needed (see note)
  • 1 tomato, diced
  • 1 large avocado, diced in ½ inch cubes
  • 1 lime. cut in wedges
  • Chopped cilantro, as needed
  • Grated white cheddar cheese, optional
  • Whole milk plain yogurt, optional
  1. Heat the oil in a heavy bottomed skillet. Add the onions and cook over medium-high heat, stirring occasionally, until most of the moisture evaporates, 15 to 20 minutes.
  2. Reduce the heat to medium-low. Continue to cook, stirring occasionally, until the onions are very soft and a caramel brown color, about 1 hour. The onions will have reduced to about 1 ½ cups. (see note)

  3. Stir in a few pinches of salt. Set aside. (If you wish, cook the onions ahead of time, saving them for later, or the next day.)
  4. Preheat oven to 350° F. Spray both sides of the tortillas with cooking spray. Toast in the oven until crisp, 10 minutes or so.
To assemble:
  1. Distribute the caramelized onions, using your fingers to spread them around on the tortillas. Sprinkle with shredded arugula. Spoon some salsa over the arugula, and then scatter with diced tomatoes and avocado.
  2. Squeeze a lime wedge on top of the vegetables and sprinkle with chopped cilantro. If dairy is in your diet, finish with cheese and drizzles of plain yogurt.
Recipe Notes
  • Prep ahead: Caramelize the onions ahead and you can have these tostadas ready in 20 minutes.
  • Look for Walla-Walla, Vidalias, or other “sweet” onion varietals. Sweet onions caramelize well because of their higher percentage of natural sugar.
  • I used the medium (4 mm) slicing blade that came with my food processor to slice the onions. I got a little teary-eyed, but not near as much as I would have slicing the onions by hand.
  • Total raw onion weight needed—2 ½ pounds, which should yield about 8 cups sliced onions. That seems like a lot--but by the time the onions are caramelized you will wonder if you have enough.
  • Substitute a different peppery or bitter green for the arugula, or just use shredded lettuce.
  • If the onions start to brown too quickly, turn down the heat; don’t hurry the slow-cooked caramelization.
  • My homemade tomato salsa recipe here.


  • Patricia

    Looks like a yummy recipe. We’re home and feeling very melancholy about the destruction in Baja and specifically feeling sad for Jim and Cita and what they have lost. Odd that this would occur to them and Andy and Nick having a fire in their house only a week before. Maybe carmelized tacos can sooth the sorrow. Love You Both, Patricia Reply · 23 September, 2014

    • Letty

      Thanks Patricia. Yes, Cynthia and Jim are in our hearts. I am putting together a list of where to donate aid $ for Baja. Reply · 23 September, 2014

  • Mona

    I am headed to the store to get the ingredients for the caramelized onion tacos with healing thoughts flowing through me and out to Baja. Love this Letty and thanks for all the magical recipes which are truly a form of art. Reply · 23 September, 2014

    • Letty

      How did Todos Santos “weather” Odile? Reply · 23 September, 2014

      • Mona

        Lots of damage, but not as bad as Cabo. Some friends lost everything and others faired pretty well. Electricity is still out in town, and the roads are policed to prevent looting. We had quite a bit of water but amazingly the palapa stayed in tact. Tough for those who are there with much recovery in the forecast . Reply · 23 September, 2014

  • Everything about these tostadas is making my mouth water! I am obsessed with caramelized onions, and arugula is one of my favorite greens! These are definitely a must-make!! Reply · 24 September, 2014

    • letty

      Thanks Julia,
      We got another huge Walla Walla (and arugula) in our CSA box this week–I’m making these again! Reply · 24 September, 2014

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