Tropical Quesadillas

1 tablespoon butter
3/4 cup chopped red onion
1 cup chopped red bell pepper
2 1/2 cups (10 ounces) shredded Monterey jack cheese
1 large mango, peeled, thinly sliced and cut into 1-inch pieces
6 (8-inch) flour tortillas

Preheat oven to 250 degrees F.

In a large skillet, melt butter over medium heat and saut