Saute the diced peppers and onion until tender with the olive oil. Add fajita mix and shrimp to the pan, cook through.
Coat one side of each tortilla with cooking spray. Place, sprayed sides down, on a cutting board or parchment paper. Sprinkle 1/4 cup of the cheese over half of each tortilla. Top with pepper, onion, and shrimp mixture. Fold tortillas in half, pressing gently.
Heat a 10-inch nonstick skillet over medium heat for 1 minute. Cook quesadillas, 2 at a time, over medium heat for 4 to 6 minutes or until light brown, turning once. Remove quesadillas from skillet; place on a baking sheet. Keep warm in a 300 degree F oven. Repeat with remaining quesadillas. To serve, cut each quesadilla into 3 wedges. If desired, serve with salsa. Makes 9 appetizer servings (2 wedges per serving).