Prepare Ingredients: Cut the onion into slivers and slice the bell peppers.
Season Chicken: In a bowl, combine 1 tablespoon of olive oil, juice of ½ lime, chili powder, paprika, onion powder, black pepper, cumin (if using), and salt. Cut the chicken breasts into strips and toss with the spice mixture.
Cook Chicken: Heat 1 tablespoon of olive oil in a pan over medium-high heat. Add half of the chicken strips and cook until just cooked through, about 3-5 minutes. Remove from the pan and set aside. Repeat with the remaining chicken.
Cook Vegetables: Add the remaining tablespoon of olive oil to the pan. Drain the onions well and add them to the pan, cooking for 2 minutes. Add the sliced bell peppers and cook for an additional 2 minutes, or until just hot.
Combine and Serve: Add the cooked chicken back to the pan with the vegetables and stir to combine. Squeeze additional lime juice over the mixture. Serve the chicken fajitas over warm tortillas and enjoy!