These super-stuffed burritos are filled with strips of grilled steak and meaty portobello mushrooms.
Heat gas grill to medium-high or prepare charcoal grill with medium-hot coals or heat broiler or grill pan over medium-high heat.
Place mushroom caps on a sheet of waxed paper, and coat tops with nonstick cooking spray.
In small bowl, mix together chili powder, salt and pepper. Liberally spread chili rub onto mushroom caps. Pat chili rub into skirt steak.
Place mushroom caps on grill. Grill 5 minutes (with grill closed). Uncover grill and flip over mushrooms. Add steak to grill. Cook, covered, 3 minutes. Flip over meat and grill an additional 2 minutes.
Remove both meat and mushrooms from grill to a cutting board; let stand 3 minutes. Slice steak and mushrooms into thin strips.
Place 1 tortilla on a plate. Spread about 1/4 cup refried beans down center of tortilla. Microwave on high for 30 to 40 seconds or until warm to the touch.
Top beans with 1/3 cup of steak-mushroom mixture, 1/2 cup shredded lettuce, 2 tablespoons taco sauce and 3 tablespoons chopped tomato. Fold up, envelope style, to enclose filling. Cover with foil to keep warm while preparing remaining burritos. Serve warm with any additional taco sauce on the side.