On a large rimmed sheet pan, toss the bell peppers and onions with 1 tablespoon olive oil and 1 tablespoon fajita seasoning until everything is coated (I like to use my hands for this).
Mix the remaining oil and seasoning in a medium sized bowl. Add shrimp and stir until shrimp is well-coated. Let marinate in refrigerator while peppers and onions roast.
Place pan in preheated oven and bake for 20 minutes.
Add shrimp, stir, and bake for 8-10 minutes more or until shrimp is cooked through and looks pink and opaque. Meanwhile, warm tortillas if desired, and prepare toppings.
Serve immediately on warm tortillas with your favorite toppings.
Make a double batch: Double the ingredients and use two sheet pans so they aren't over-crowded.
I love to use corn tortillas but flour tortillas are great with this recipe. You can wrap them in foil and add them to the oven in the last 5 minutes of cooking time for nice warm tortillas. For corn tortillas, if desired, char them slightly over a gas flame burner using tongs.
Nutrition information is for two fajitas in corn tortillas, and does not include toppings.