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.