Wash the potatoes and beetroot. Cook separately in boiling water for about 15 minutes. Meanwhile peel and finely dice the onion. Knead minced meat, egg yolk, breadcrumbs and onion. Season with salt, pepper and mustard.
Shape into 4 meatballs. Drain the potatoes and beetroot, cut in half and scratch several times. Spread small bay leaves and thyme on top. Sprinkle with coarse salt and pepper. Spread everything on a baking tray lined with baking paper.
Cook in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for 20-25 minutes. In the meantime, wash the chives and parsley and chop finely, except for a little parsley for garnishing.
Mix the quark and herbs, except for some chives. Season to taste with salt, pepper and lemon juice. Arrange everything in a casserole dish. Garnish with parsley.