Mix fajita seasoning, olive oil, and garlic together in a large bowl. Add shrimp, peppers, and onion to the bowl and toss to coat.
Evenly spread shrimp, peppers, and onions in a single layer on a baking sheet.
Cook in the oven until the shrimp is cooked through, about 6-8 minutes. Remove the shrimp and set aside.
Set the oven to broil, and cook remaining peppers and onions until they begin to char, about 2 to 3 minutes. Remove from the oven and serve immediately with the shrimp.
Notes
We usually use frozen shrimp in this recipe, but make sure you thaw it before cooking. You can do that overnight in the refrigerator or under running water for a few minutes. Just make sure to pat the shrimp dry before adding it to the recipe.