Toss the shrimp with fajita seasoning until evenly coated.
Heat butter or oil in a skillet over medium heat. Add the bell pepper and onion, and sauté until tender and lightly browned.
Add the garlic and shrimp to the skillet. Cook 2 to 3 minutes, until the shrimp are pink and cooked through.
Place a tortilla in a clean pan. Layer cheese, the shrimp fajita mixture, and more cheese. Top with another tortilla or fold it over.
Cook 2 to 3 minutes per side, until golden brown and the cheese is fully melted.
Cut into wedges and serve hot with lime wedges, sour cream, or salsa.