Place onion and green pepper in a small microwave-safe bowl. Cover, venting one corner, and microwave on high for 3 to 4 minutes. Stir in 3 tablespoons of the salsa; set aside.
Divide cheese evenly among the tortillas. Arrange beef over cheeses and top with vegetable mixture. Fold tortillas over to close.
Heat a medium nonstick skillet over medium heat for 5 minutes. Heat two quesadillas in pan about 2 minutes, turning once, to melt cheese and warm filling. Repeat with remaining quesadillas. Serve with remaining salsa.
A meal that is nothing but comfort food and long may it last. I love this recipe.