Delia smith has a recipe for caramelised balsamic red onion and goats cheese tarts in the vegetarian collection book. Basically you cook a kilo of finely sliced red onions in butter and balsamic vinegar (not sure of amount) for about 40 mins. I use ready made puff pastry cut into individual tart size squares, top with the cooked onions and a slice of goats cheese and bake.
They're yummy - we're having them for starters on Christmas day.
I always make double the quantity of onions as they freeze well in small portions and are good to serve with sausages, on pizzas, in stews etc