Spread onions in shallow baking dish. Dot with butter; sprinkle with brown sugar, vinegar, salt and pepper to taste.
Bake, covered, in 300' F oven for 30 to 40 minutes or until tender, stirring once. (Can be prepared ahead, covered and refrigerated for up to 2 days.).
Place pita rounds on baking sheets. Top each round with a spoonful of onion mixture, cheese and 2 to 4 slivers sundried tomatoes. Just before serving, bake pizzas in 375' F oven for 6 to 8 minutes or until lightly browned and bubbly. Makes about 36 pizzas.