Bring vegetable broth to a boil in a saucepan. Reduce the heat, cover, and keep at a simmer.

Meanwhile, heat olive oil and butter in a heavy-based saucepan over medium heat. Add onion and cook until golden. Reduce heat to low, then add arborio rice and cook, stirring, until the rice is lightly golden (around 3 minutes).

Pour in about 1/4 of the hot stock and stir constantly until the liquid has been absorbed. Repeat this process until all the stock has been added and the rice is tender. This will take around 20 minutes.

Stir in the grated cheese and remove from heat. Transfer the mixture to a bowl, allow to cool, then refrigerate for an hour.

With wet hands, roll 2-3 TBSPs of the rice mixture into a ball. Push a cube of mozzarella and a few pieces of sun-dried tomato into the ball, reshaping to cover up the cheese and tomato. Repeat with remaining rice mixture to form 18 balls. Refrigerate for another 15 minutes.

Heat the vegetable oil in a heavy-based saucepan over medium low heat. Using a large slotted spoon, lower 1/2 of the risotto balls into the oil. Cook for 1-2 minutes or until golden brown, then flip over. Remove with the spoon, and drain on paper towels. Repeat this process with the remaining cakes. Serve immediately.