Preheat oven to 425 F.
Slice onion and bell peppers into strips. Slice limes and peel garlic cloves, leaving whole. Spread vegetables and limes on a non-stick baking sheet and drizzle with olive oil. Turn to coat.
Bake for 15 minutes then remove from oven. Remove limes and garlic and set aside.
In a food processor, blend olive oil, cilantro, fajita seasoning, roasted garlic and squeeze the juice from the limes into the food processor. Use tongs to squeeze limes being careful not to burn yourself. Blend until well blended.
Pour marinade onto chicken and mix well until coated evenly. Spread chicken onto spreadsheet evenly. Return to oven with veggies and bake for an additional 12-15 minutes. Turn chicken and veggies halfway.
Remove from oven and serve with your favorite toppings and sides!