Preheat the oven to 425 degrees and line 2 baking sheets with parchment paper.
Shred the mushrooms and toss with 1 tablespoon oil and 1 tablespoon taco seasoning on a baking sheet. Spread evenly. On another baking sheet, toss the onion, bell pepper, remaining oil, and taco seasoning, then spread evenly. Roast both sheets at 400°F for 15 minutes, toss, and roast for another 5 minutes.
Mix the mayo, salsa verde, and lime juice in a bowl.
Heat tortillas on the stove-top: use a dry pan for 30 seconds per side or char directly over a gas flame. Brush one side of each tortilla with oil, flip, and spread 1 tablespoon of sauce on the other side. Add roasted veggies, fold, and place on a baking sheet. Repeat with all tortillas.
Bake tacos at 400°F for 10 minutes, flip, and bake another 5-7 minutes. Serve hot with your favorite toppings.
Serve and enjoy!