Melt 2 tablespoons butter in medium skillet over medium heat. Add onion,
brown sugar, and vinegar; saute until onion is golden brown, stirring
frequently, about 25 minutes. Remove from heat. Cool to room temperature.
Preheat oven to 350 degrees. Sprinkle cheese over half of each tortilla,
dividing equally. Sprinkle prociutto and sauted onion over cheese. Season
with pepper. Fold other half of tortilla over cheese mixture. Brush
tortillas with some of melted butter.
Brush heavy large skillet with some of the melted butter. Place over
med-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 hevy 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.
NOTE: I sometimes speed up the process by placing prepared quesadillas
directly under the broiler for a quick minute or so on each side.
