1. Heat the butter and oil; sauté the onion, celery and garlic until glossy. Add the rice and stir until all the grains are coated with oil and lightly sautéed.
2. Add the wine and boil rapidly until all the wine has been absorbed. Add the hot stock, a spoonful at a time, stirring occasionally until the rice is al dente. This takes about 35 minutes at a fairly high heat. Remove from the heat, stir in the Parmesan, biltong and lemon juice, then spread the hot rice out in a flat dish so that it can cool quickly and doesn’t continue steaming.
3. Roll the cooled rice into walnut-sized balls. Flatten the balls one by one in the palm of your hand, place a block of mozzarella on top and shape the rice around it until completely covered. Try to keep the layer of rice as thin as possible.