Pre-heat oven to broil and line a baking sheet with foil.
Toss onions, peppers, and garlic together with 1 Tbsp olive oil, ½ tsp salt, and ½ tsp pepper and lay in a single layer on lined baking sheet. Broil for 6-10 minutes until vegetables are crisp-tender.
While veggies are in the oven, in a medium bowl mix together chili powder, cumin, garlic powder, ¼ tsp salt, and ¼ tsp pepper. Add the chicken breasts to the bowl and toss to coat with spice mixture.
Heat 1 tablespoon olive oil in a large skillet over medium-high heat. Add the chicken and cook, turning, until golden and cooked through, 3-4 minutes per side. Remove from pan to a cutting board and let cool slightly before slicing.
In a large bowl, toss veggies and sliced chicken together.
Wrap tortillas in a damp paper towel and heat in the microwave for about 30 seconds-1 minute. Top tortillas with chicken and vegetable mixture and add any other toppings desired.