Smoked Gouda and Caramelized Onion Quesadillas

Source: Bon Appetit magazine - September 1995

2 tablespoons butter
1 onion, thinly sliced
1 tablespoon golden brown sugar
1/4 teaspoon white wine vinegar
1 1/2 cups grated smoked Gouda cheese
4 (10-inch diameter) flour tortillas
2 ounces sliced prosciutto, chopped
Pepper to taste
2 tablespoons butter, melted

Preheat oven to 350 degrees F for 15 minutes before baking.

Melt 2 tablespoons butter in heavy medium skillet over medium heat. Add onion, brown sugar and vinegar; saut