In a small bowl, mix chili powder, cumin, paprika, red pepper flakes, salt, and black pepper.

Rub the spice mixture evenly over both sides of the steak.

Heat 1 tablespoon of olive oil in a large skillet over medium-high heat. Add the steak and cook for about 5-7 minutes on each side for medium-rare, or until desired doneness. Remove from skillet and let rest.

In the same skillet, add the remaining tablespoon of olive oil. Add bell peppers, onion, and garlic. Sauté for 5-6 minutes, or until vegetables are tender.

Slice the steak against the grain into thin strips. Return the steak to the skillet with the vegetables. Squeeze lime juice over the mixture and stir to combine.

Warm tortillas according to package instructions.

Serve the steak and vegetable mixture on warm tortillas, garnished with avocado, sour cream, shredded cheese, and cilantro if desired.