• Very lightly oil a 12 x 16-inch baking sheet. Combine the water, vegetable broth, salt, polenta and the olive oil in a cooking container, either a pressure cooker, microwave-safe bowl, or heavy soup pot. 
• To cook in an Instant Pot pressure cooker: Put the vegetable broth and water, polenta, olive oil, and salt in the Instant Pot. Do not stir. Lock lid. Sent the top Vent to seal. Cook on Porridge setting with time adjusted to 7 minutes. Natural pressure release for 10 minutes, then vent to release the pressure completely.
• To cook in microwave oven: Cover and cook on highest power for five minutes. Stir, cover again and cook for another five minutes. Stir in the nutritional yeast, cover and cook for three more minutes. 
• To cook stovetop: Bring 2 cups water and 2 cups vegetable broth to a boil in a medium-size heavy sauce pan over high heat. Add the salt and olive oil. Slowly pour cornmeal into the hot liquid, stirring with a wire whisk or wooden spoon. In 2 to 3 minutes, when the polenta begins to thicken, reduce heat to low. Cook for at least 45 minutes, giving it a stir every 5 minutes or so. When it is very thick, stir in the nutritional yeast.
• Quickly pour the soft polenta into the prepared baking sheet and spread into an even layer before it cools. It will be a bit less than ¼-inch thick. Cover and chill until firm.
• To crisp the polenta: Preheat oven to 450 °F. Turn the polenta out onto a cutting board and cut into 20 portions, approximately 3-inch square. Line the baking sheet with parchment paper. Arrange the cut squares on the parchment and brush them lightly with olive oil. Bake 25 to 30 minutes, until you see a hint of golden brown around the thinnest edges. Remove from the oven but keep warm.

While the polenta crisps in the oven, make the gravy topping:

• Heat the olive oil in a large skillet. Sauté the onions and mushrooms over medium-high heat, until the onions and mushrooms are just taking on color and the liquid has evaporated. Add the garlic and cook another minute. Lower the heat to medium and stir in the arrowroot or flour, stirring to coat the mushrooms and onions.
• Add the vegetable broth, stirring continuously for about a minute, until you see the sauce thicken a wee bit. Add the tahini, beans, sage, and salt. (see note) Stir another minute or so. Resist the impulse to cook it longer as tahini sauces thicken on their own, with time. If needed, stir in 1/4 cup more vegetable broth. Season with 20 grinds of pepper and more salt, to taste.
• To serve: Arrange 3 polenta squares on each plate and top with a generous spoonful of the mushroom and bean gravy. Sprinkle with parsley.