Fry the onion in olive oil until it turns translucent, add the rice and fry it well.
Add boiling water to cover the rice and braise on low heat until soft.
If necessary, keep adding hot water, so that the rice is covered at all times.
Bring the balsamic vinegar with sugar to a boil and cook it until it turns into gel. When the rice is cooked, place it on a baking try, pour the vinegar syrup over it, season with salt, mix and let it cool.
Cut the carrots into julienne (strips) and cut the tuna into thin slices.
Put a fist of cooked rice onto a plastic foil as if you were making sushi, place the tuna in the middle of the rice and different carrots on each side.
Grab the edges of the foil and wrap the dish into a roll, then leave to cool in the fridge for about two hours.
Unwrap the cooled roulade and cut it into smaller slices.
