In a small bowl combine olive oil, salt, and pepper.
Brush the portobello mushrooms and sweet pepper with the olive oil mixture. Stack tortillas and wrap in foil.
For a charcoal grill, place the portobellos, pepper quarters, and foil packet on the rack of an uncovered grill directly over medium coals. Grill 8 to 10 minutes or until tender, turning once halfway through grilling time. (For a gas grill, preheat grill. Reduce heat the medium. Place portobellos, pepper quarters, and foil packet on a grill rack over heat. Cover and grill as above.)
Slice portobello and sweet pepper into strips. In a medium bowl, mash one of the avocados; stir in mayonnaise and chili powder. Season to taste with salt and pepper. Slice remaining avocado. Serve sweet pepper, portabello and sliced avocado on tortillas. Top with mayonnaise mixture. Makes 4 servings.