Preheat oven to 375 degrees. Line half sheet pan with parchment paper or foil and set aside.
Trim any extra fat or cartilage off of chicken and slice across the grain into skinny strips. Place chicken, peppers, and onions onto the baking sheet. Toss with olive oil, seasoning, and salt.
Bake 20-30 minutes or until chicken is thoroughly cooked and onions are tender.
Serve chicken and peppers with tortillas and any extra toppings desired.
In a large nonstick skillet with high sides, heat 2 tablespoons olive oil over medium-high heat. Add onions, chicken and 2 tablespoons fajita seasoning. Stir as onions brown and chicken cooks through. After 5 minutes, the chicken should be cooked through and onions should be crisp-tender. Remove from pan.
Place pan back on the heat, add in other 2 tablespoons olive oil, peppers and last tablespoon of fajita seasoning. Fry until crisp-tender, about 3 minutes.
Add in chicken and onions to the pepper mixture and stir to heat everything through another minute or two.
Remove from heat and serve hot with tortillas and any extra toppings desired.