1. Prepare the broth: Heat the water in a pot and add the bouillon cube to make 4 1/4 cups (1 liter) of hot broth.

2. Sauté the onion: In a saucepan, heat the olive oil over medium heat. Add the chopped onion and cook until translucent.

3. Add the rice: Stir the rice into the saucepan and cook for about 2 minutes, until it becomes slightly translucent.

4. Cook the risotto: Begin adding the hot broth one ladle at a time, stirring constantly. Let the rice absorb the broth before adding the next ladle. Repeat this process for about 18-20 minutes, until the rice is creamy and al dente.

5. Add your tomato sauce mixture to the risotto and stir gently.

5. Prepare the tomato sauce: Cut the tomatoes into wedges, then add them to a skillet with a small drizzle of olive oil. Add the tomato purée, and season with salt and pepper to taste.

6. Remove the saucepan from the heat and add the butter and Parmigiano Reggiano. Stir well to create a creamy texture. Then add the mozzarella balls, cut into pieces, and season with salt and pepper.

7. Serve hot, garnished with fresh basil if desired.