Preheat the oven to 400ºF. Arrange onion, bell peppers, mushrooms, and corn in a single layer across one or two baking sheets. Sprinkle with salt and pepper to taste and toss with 1 tablespoon olive oil. Roast in the oven until the veggies begin to brown and caramelize, stirring two or three times, about 30 minutes.
When the vegetables come out of the oven, arrange the corn tortillas in a single layer on a baking sheet (working in batches as necessary) and lightly spray or brush both sides with olive oil. Bake the tortillas until crisp and just beginning to brown, 10-15 minutes.
While the tortillas are cooking, toss the roasted vegetables with the black beans and adjust seasonings as necessary. When the tortillas are done, top with the vegetable mixture. Finish with a tablespoon of cheese, avocado, tomatoes, microgreens, cilantro, a squeeze of lime juice, and a drizzle of poblano cream sauce if desired.
Tips for customizing
Use my oven roasted corn and black bean salsa in place of the roasted veggies
Drizzle with green avocado sauce, chipotle enchilada sauce, or top with healthy guacamole
Leave off the cheese and poblano cream sauce to make these vegan veggie tostadas
Use whatever vegetables you have on hand such as cauliflower or zucchini, adjusting the cooking time as needed
Swap the Monterey jack cheese for crumbled queso fresco