Peel and finely dice the onion and garlic. Clean, wash and chop the peppers. Heat the oil in a frying pan, fry the onion, garlic and bell peppers in it while turning over medium heat for about 3 minutes. Add tomato paste and ketchup, stir in and let it boil down again for about 2 minutes while stirring. Season to taste with salt, pepper, paprika, cayenne pepper and sugar.
Remove from the stove and let it cool down. Wash the basil, carefully dab dry, pluck the leaves from the stalks and cut into fine strips. Stir basil into the paprika mixture. Drain the mozzarella, cut into the upper third and fill with the paprika-basil mixture, sprinkle with pepper. Serve garnished with basil leaves as desired