Instructions: Preheat oven to 350 degrees 15 minutes before baking.

Melt 2 tablespoons butter in heavy medium skillet over medium heat. Add onion, brown sugar and vinegar; saute until onion is golden brown, stirring occasionally, about 25 minutes, lowering heat if necessary to prevent over-browning. Remove from heat. Cool to room temperature.

Sprinkle cheese over half of each tortilla, dividing equally. Sprinkle prosciutto and sauteed onion over cheese. Season with pepper. Fold other half of each tortilla over cheese mixture. Brush tortilla with some of melted butter.

Brush heavy, large skillet with some of melted butter. Place over medium-high heat. Working in batches, cook quesadillas just until brown spots appear, brushing skillet with butter between batches, about 2 minutes per side. Transfer quesadillas to heavy large baking sheet. Bake until tortillas are golden and cheese melts, about 5 minutes.

Transfer quesadillas to work surface. Cut each into 6 triangles. Arrange on platter and serve hot.

If you would like to email yourself the recipe for later use, or share the recipe with your friends or family, enter the email addresses below and this recipe will be emailed to you and others as well.