Put the mushrooms, onions and peppers in a large resealable bag. Combine the oil, vinegar, chili powder, salt and garlic, and add it to the bag with the mushrooms. Massage the vegetables gently to coat well with the marinade. Refrigerate for at least 30 minutes and up to 48 hours.

Preheat the grill to medium-high heat. Spray a vegetable grilling tray or a piece of foil with nonstick cooking spray, or brush it with a little vegetable oil, and heat it on the grill. Drain the vegetables in a colander, then transfer them to the grilling tray. Grill them with the cover closed for about 15 minutes, flipping them once, until they are tender and partially browned but not charred.

Heat the tortillas in the microwave for 30 seconds to 1 minute so they are soft and warm. Lay about ½ cup of the vegetable filling on the center of each tortilla, top it with about 2 tsp. cheese, and about 1 Tbsp. each sour cream and salsa, if desired. Fold the bottoms up and the sides in and serve them immediately.