Peel and wash the potatoes, cover and cook in salted water for about 20 minutes. Soak rolls in cold water. Line a baking tray with baking paper. Peel and finely dice onions. Squeeze the bread roll.
Knead the ground pork, minced meat, onions, bread rolls, eggs, mustard, 2 teaspoons salt and 1 1/2 teaspoons pepper. Form 12 meatballs with moistened hands and place them tightly together on one half of the baking tray.
Preheat oven (electric cooker: 225°C/circulating air: 200°C/gas: level 4). Drain potatoes. Add 4 tablespoons butter. Close the pot with the lid and toss vigorously so that the potatoes break up a little.
Spread the potatoes over the remaining surface of the baking tray. Bake the meatballs and potatoes together in a hot oven for 30-35 minutes.
In the meantime, stir cream cheese, crème fraîche and mineral water until smooth. Season to taste with salt and pepper.
Wash the parsley, shake dry and place in a tall mixing bowl. Add vinegar and oil. Puree with a chopping stick. Wash half of the apple, remove seeds and dice very finely. Stir under the parsley pistou.
Season with salt and pepper.
About 10 minutes before the end of the baking time, spread the cream of fresh cheese on the meatballs and finish frying. Season the potatoes with coarse salt. Arrange potatoes, meatballs and parsley pistou.