Heat half the oil in a medium frying pan on low. Cook onion for 15 minutes, covered or until very soft. Add sugar and vinegar and stir until combined. Cook for 5 minutes until onion is caramelised
Meanwhile, heat remaining oil in a large saucepan on high. Add rice and cook for 1 minute, or until rice is coated.
Add all but ¼ 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 (I left this out of mine as I thought that would be too much liquid) with butter, ¾ cup of parmesan and caramelised onion and stir to combine. Transfer to a container and refrigerate overnight.
Preheat oven to 180C (360F) fan-forced.
Grease a 24-hole mini muffin tin (or use a baking paper lined tray).
Roll 1 tablespoon of risotto into a ball. Press a hole in the centre of the ball, fill with a piece of mozzarella and carefully roll ball to enclose. Press into prepared muffin tin (or place on tray). Repeat. Sprinkle remaining parmesan over risotto balls.
Bake for 20 minutes or until firm and golden. Cool in pan for 5 minutes. Run a small knife around the edge of cakes to loosen before removing from the pan.