In a large skillet, heat the olive oil over medium heat. Add the diced red onion and bell pepper, and sauté for about 5 minutes until softened.
Add the minced garlic, cumin, chili powder, salt, and pepper. Cook for another minute until fragrant.
Stir in the black beans, pinto beans, and corn. Cook for about 5-7 minutes, stirring occasionally, until heated through.
Warm the tortillas in a separate pan or microwave until pliable.
To assemble, place a generous scoop of the bean mixture in the center of each tortilla. Top with shredded lettuce, diced tomatoes, avocado slices, and salsa. Add cheese if desired.
Fold in the sides of the tortilla and roll it up tightly from the bottom.
Serve immediately or wrap in foil for later.