In a bowl, mix the turkey with cumin and garlic powder. Set aside.
Combine the cheddar and Monterey Jack cheeses in another bowl.
Lay out 4 tortillas on your work surface. Sprinkle each with a quarter of the cheese mixture, then top with turkey, and add peppers and onions if using.
Top each with another tortilla, pressing down gently to seal.
Heat a large skillet over medium heat and add a pat of butter. Once melted, place one quesadilla in the pan.
Cook for 2-3 minutes until golden brown on the bottom, then carefully flip and cook another 2-3 minutes until the cheese is melted and the other side is golden.
Remove from pan and repeat with remaining quesadillas, adding more butter as needed.
Cut each quesadilla into 4 wedges and serve immediately with sour cream and salsa.