Soak rolls in water. Peel onions, finely dice one onion. Squeeze out the bread roll well, knead well together with minced meat, ground pork, diced onion, mustard and eggs. Season with salt and pepper. Form 19 meatballs from the mixture. Put them together on a baking tray to form a "sun".
Cut an onion into thin slices and spread on some of the outer meatballs. Wash, peel and roughly grate the potatoes. Mix with egg yolk and salt. Distribute on the two inner rings of the chopping sun. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for about 40 minutes