3/4 to 1 pound pork tenderloin
2 tablespoons butter, divided
2 medium onions, sliced
1/2 teaspoon salt
1/4 teaspoon ground black pepper
2 tablespoons sugar
1/4 cup port wine or sweet sherry
2 tablespoons balsamic vinegar
1 cup fresh or frozen blueberries
1 cup chopped cherry tomatoes 

Preheat broiler. Broil pork, turning occasionally until cooked through, about 20 minutes. Remove to a platter; cover to keep warm.

Meanwhile, in a large skillet over medium-high heat, melt 2 tablespoons of the butter. Add onions, salt and pepper; cook until onions are golden, about 10 minutes. Add sugar; cook until onions are caramelized, 3 minutes longer. Add port, balsamic, blueberries and tomatoes and bring to a boil. Remove from heat. Thinly slice pork and serve with sauce.