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Caramelized Onion and Arugula Tostadas
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5 from 2 votes

Caramelized Onion and Arugula Tostadas

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.
Prep Time15 mins
Cook Time1 hr
Total Time1 hr 15 mins
Course: Appetizer, Main Course
Cuisine: Mexican, Vegetarian
Keyword: vegetarian, tostadas, caramelized onions
Servings: 4 6-inch tacos or 10 (4-inch) tostadas.
Calories: 208kcal


  • 2 1/4 pounds sweet onions, thinly sliced, 3 large or 5 smaller onions (see note)
  • 2 tablespoons olive oil
  • sea salt
  • 4 (6-inch) corn tortillas or 10 (4-inch) corn tortillas
  • cooking spray, as needed
  • 1 ½ cups arugula, chopped (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
  • Guacamole avocado lime sauce optional


  • 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.
  • 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)
  • 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.)
  • 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:

  • 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.
  • 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.


  • 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.
  • Make them vegan: Skip the cheese and yogurt--drizzle your tostadas with guacamole avocado sauce.
  • My homemade tomato salsa recipe here.


Calories: 208kcal | Carbohydrates: 32g | Protein: 4g | Fat: 8g | Saturated Fat: 1g | Sodium: 36mg | Potassium: 453mg | Fiber: 4g | Sugar: 14g | Vitamin A: 434IU | Vitamin C: 18mg | Calcium: 87mg | Iron: 1mg