Heat 1/2 the oil in a medium frying pan on low. Cook onion for 15 minutes, covered, or until very soft. Add the sugar and vinegar and stir until combined. Cook for 5 minutes, until onion is caramelized.
Meanwhile, heat remaining oil in a large saucepan on high. Add the rice and cook for 1 minute or until rice is coated. Add all but 1/4 cup of hot stock. Stir and simmer on low, covered, for 12-15 minutes, or until rice is tender. Remove from heat, add remaining stock with butter, 3/4 cup of the Parmesan and the caramelized onion; stir to combine. Transfer to a container and chill overnight.
Preheat oven to 200^ . Grease a 24 hole mini-muffin pan. Place a circle of waxed papen in each mini well;spray paper. Roll 1 Tablespoon or the risotto into a ball. Using index finger, press a hole in center of ball, fill with a piece of mozzarella and roll ball to enclose the cheese. Press into well sprayed mini-pan. Repeat with remaining risotto and cheese. Sprinkle remaining Parmesan cheese over the rice balls.
Bake for 20 minutes , or until firm and golden. Cool in pan for 8 minutes, Run a small spatula of knife around edge of cakes to loosen before removing from pan. remove waxed paper. Serve warm or cold.