Preheat oven to 180 degrees celsius.
Place leftover risotto into a bowl (it's best if it's been left overnight in the fridge), add the egg and breadcrumbs and mix to combine.
Add more breadcrumbs if your mixture is too soft (it needs to be quite hard so that the balls keep their shape).
Return the mixture to the fridge for 30 minutes.
Place the panko crumbs into a bowl and use your fingers to mix through the olive oil, so that the crumbs are completely coated.
Roll 2 tablespoon-sized scoops of risotto mixture, flatten out and place a cube of mozzarella cheese in the centre. Roll back into a ball to completely enclose the cheese.
Place into the bowl of panko crumbs and cover completely.
Place each of the balls into the hole of a greased mini-muffin tin.
Bake for 30 minutes or until golden.
Serve with tomato relish.