Wash the leek and cut it into thin rings. Peel the potatoes and cut them into even pieces so they cook evenly. Clean the Brussels sprouts (halve larger ones if needed).

Beat the eggs. Mix the pork and beef mince with the beaten egg, the mustard, and half of the sliced leek. Knead briefly and shape into even meatballs with damp hands.

Place the potatoes in a large pot with cold water and bring to a boil. Once the water is boiling, cook the potatoes for a total of about 15 minutes.

While the potato water is heating, heat the unsalted butter in a large frying pan over high heat. Fry the meatballs for about 3 minutes over high heat so they brown all over. Then lower the heat and let the meatballs cook gently for a total of about 20 minutes. Turn them regularly so they cook evenly.

After the potatoes have been boiling for about 8 minutes, add the Brussels sprouts to the pot. In the last minute of cooking, add the remaining leek. Check with a fork whether the potatoes are tender before draining.

Drain the potatoes, Brussels sprouts, and leek and reserve about 1/2 cup of the cooking liquid per 2 servings. Return the cooked vegetables to the pot or a large bowl, add the cottage cheese and the reserved cooking liquid, and mash with a potato masher until smooth and creamy.

Season the mash with pepper and salt. Divide the mash and the meatballs over the plates and serve with extra mustard to taste. Enjoy!