Roasted vegetables make fine fillings for meat-free tacos; besides the ones used here, try corn, bell peppers, other types of mushrooms, and any type of squash or potato.
Preheat oven to 425 degrees. On a rimmed baking sheet, toss mushrooms with 1 teaspoon oregano, 1 tablespoon olive oil, and the water; season with salt and pepper. On another baking sheet, toss zucchini and onion with remaining oregano and olive oil; season with salt and pepper.
Roast, tossing occasionally, until vegetables are browned and fork-tender, 25 to 30 minutes (zucchini may cook faster than mushrooms).
Using tongs, hold each tortilla directly over a gas flame, turning, until heated through, about 5 seconds. (Alternatively, wrap tortillas in parchment, then foil, and warm in a 350 degrees oven.)
To serve, fill each tortilla with mushrooms, vegetable mixture, shredded cheese, and tomatoes.
New Take on Tacos: Substitute hearty roasted portobello mushrooms and zucchini for the ground beef. Then top with shredded cheese and chopped vegetables.
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