For the rolls mix yeast, 1-2 teaspoons salt and flour. Add 300 ml lukewarm water and knead to a smooth dough. Cover and leave to rise in a warm place for about 1 hour until the dough has doubled in size.
Meanwhile peel onions and garlic and cut them into fine cubes. Put the tomatoes in a bowl and dice them roughly. Put pretzels in a freezer bag and crush them into crumbs with a rolling pin.
Drain a mozzarella ball and cut into approx. 24 cubes. Put the minced meat, mustard, pretzel crumbs, egg, 4 tbsp. cold water, garlic and approx. 1/3 onions in a bowl, season well with salt and pepper and knead.
Form approx. 24 meatballs from the minced mass, press each flat, place 1 piece of mozzarella on top of each and seal the minced meat on top. Heat the oil in a large pan. Fry the meatballs for about 5 minutes while turning.
Remove, fry the remaining onions briefly in the frying pan. Add tomatoes, bring to the boil and season to taste with salt, pepper and sugar. Place meatballs in the sauce and braise covered in it for about 10 minutes.
Quarter the yeast dough. Form each quarter of dough into a long roll. Place them side by side on a baking tray lined with baking paper. Cover and leave to rise in a warm place for about 1 hour. In a preheated oven (electric cooker: 200 °C/ circulating air: 175 °C/ gas: see
manufacturer) bake for about 20 minutes.
Drain the remaining mozzarella. Cut mozzarella and gouda into strips and mix. Remove rolls from the oven, let them cool slightly, cut them horizontally and open them. Spread about half of the cheese on the cut surfaces of the upper roll halves.
Spread meatballs with tomato sauce on the lower halves and sprinkle with remaining cheese.
Spread the bread roll halves on a baking tray lined with baking paper and bake in a preheated oven (electric cooker: 225 °C/ circulating air: 200 °C/ gas: see manufacturer) for approx. 5 minutes. In the meantime, wash the basil, shake it dry, pluck the leaves from the stalks and roughly pluck them.
Sprinkle Meatballs with basil, put the lid on the rolls. Add any remaining meatballs.