Preheat the oven to 400F. Line two sheet pans with parchment paper. A half sheet pan is ideal for the chicken and veggies and a quarter sheet pan is ideal for the salsa.
Slice the bell peppers into sticks and onion into wedges. Add the peppers and onions to one side of a prepared sheet pan.
Slice chicken on the bias (see this video for help), about ½-inch thick. Add the chicken to the other half of the sheet pan.
Combine the fajita seasonings in a small bowl: chili powder, salt, black pepper, oregano, smoked paprika, cumin. Rub the seasonings into the chicken on all sides and sprinkle over the vegetables. Drizzle a small amount of avocado or extra virgin olive oil on top of the chicken and veggies (just enough to lightly coat). Using your hands, mix the oil with the veggies and chicken. Make sure everything is in a single layer on the sheet pan.
Combine all the salsa ingredients, except the cilantro and lime juice, on the second sheet pan.
Bake the chicken and veggies and salsa ingredients for 20 minutes, rotating the sheet pans halfway through.
Once cooked, carefully add the salsa ingredients to a blender, along with the juice from half of a lime and cilantro. Carefully pulse the ingredients, making sure to vent for steam. You can make this salsa as smooth or as a chunky as you desire.
Squeeze the other half of the lime over the fajitas. Sprinkle chopped cilantro over the fajitas (optional). Stir the lime juice and cilantro into the fajita mixture.
Serve the fajita mixture with tortillas, sour cream, guacamole, etc. Or, serve in butter lettuce in place of the tortillas. Another alternative is to serve the chicken and veggies with rice to make fajita burrito bowls. Top with the homemade salsa. The salsa may also be made and served alone with tortilla chips--so good!
Notes
Chicken Note: If you want, you could use boneless, skinless chicken thighs in place of the chicken breasts.