1/4 cup water
1 tablespoon tomato paste
1 teaspoon chili powder, or to taste
1/8 teaspoon ground cumin
Dash of hot-pepper sauce, or to taste
1 teaspoon olive oil
1/2 cup sliced zucchini, halved lengthwise and cut into 1/4-inch slices
1/2 cup sliced mushrooms
1/4 cup coarsely shredded carrot
1 tablespoon chopped onion
1/4 teaspoon minced garlic
1 plum tomato, cut into 1/2-inch cubes (1/2 cup)
One 6- or 7-inch flour tortilla (white or whole wheat)
Shredded Cheddar cheese or freshly grated Parmesan cheese
Alfalfa sprouts or shredded lettuce
Adjust the oven rack 4 to 5 inches from the broiling element; preheat the broiler.
Stir together the water, tomato paste, chili powder, cumin, and hot-pepper sauce in a measuring cup; set aside.
Heat 1/2 teaspoon of the olive oil in a small skillet over medium heat. Add the zucchini, mushrooms, carrot, and onion. Cook, stirring, until the vegetables are tender, about 4 minutes. Add the garlic; cook 30 seconds longer. Stir in the tomato-paste mixture and tomato cubes. Reduce the heat to low, cover, and cook for about 5 minutes, stirring occasionally.
Meanwhile, brush both sides of the tortilla with the remaining olive oil. Prick the surface of the tortilla in several places with a fork. Place directly on the oven rack; broil until lightly browned, 1 to 2 minutes on each side. Watch closely! Place on a serving plate.
Taste the topping; adjust seasonings. Mound on the toasted tortilla. Garnish the tortilla with cheese. As a garnish on the plate, place a mound of alfalfa sprouts or shredded lettuce with orange slices arranged on top.
* Add or substitute other vegetables such as small broccoli florets or corn.
* Add about 2 tablespoons cooked beans such as black, kidney, or garbanzo beans.
* Add canned chopped green chilies or minced jalapeno pepper.
* In place of the tortilla, stuff the mixture into 2 taco shells; top each with lettuce and cheese; drizzle with taco sauce.
* Use the Vegetarian Tostada topping as a filling for an omelet, as a pita-bread filling, or as a topping for English muffins (sprinkle with shredded cheese, and place under the broiler for a few minutes until melted).