Heat the butter in a frying pan and fry the meatballs until golden-brown (about 5 min) on medium heat. Meanwhile, chop the garlic and onion finely.
Take the meatballs out of the frying pan and set aside. Switch to low heat, add the onion, garlic and rosemary and stir for a minute. Add the flour and stir for one more minute. Add the right amount of beer and 25 ml water per person, stir until smooth. Add the meatballs to the pan and cook, covered, on low heat for 20 minutes. Bring to taste with salt and pepper.
Heat 500 ml water per person in a pan with lid for the potatoes (soprano) and kohlrabi. Meanwhile, peel the potatoes and kohlrabi and cut them into wedges. Add to the boiling water and let it cook, covered, for 15 minutes until ready. Drain the potatoes, save a little bit of the water and keep apart.
Mash the potatoes and kohlrabi to a stew. Add a dash of milk and/or the retained water to make it a bit more creamy. Add the mustard and bring to taste with salt and pepper. Tear the purslane into smaller pieces and mix most of it with the stew.
Divide the stew and meatballs onto the plates. Garnish with the rest of the purslane and sauce.