Boil 4 eggs in boiling water for about 7 minutes until soft and waxy. Quench the eggs in cold water and peel them. Peel braised cucumbers, cut in half lengthwise, scrape out the seeds with a spoon. Sprinkle halves with a little salt and let the juice stand.
Melt the fat, sprinkle with flour while stirring and sweat on. Gradually add stock, cream and milk while stirring. Stir in mustard, up to 1 tsp. Bring sauce to the boil while stirring and season with salt and pepper.
Cut the eggs into six. Coarsely divide the sixths of 3 eggs, stir into the mustard sauce with the frozen peas. Cover the rest of the eggs and put them aside.
Dab cucumbers dry with kitchen paper. Place the halves in a flat casserole dish and fill with about half the sauce. Bake in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: see manufacturer) for approx. 30 minutes.
Peel the onion and cut into small cubes. Knead minced meat with onion, 2 eggs, 1 tsp. mustard and breadcrumbs. Season with salt and pepper, form 8 meatballs with moistened hands. Heat the oil in a pan and fry the meatballs in it at medium heat for 6-8 minutes on each side.
Wash the dill, dab dry and pluck flags from the stems. Finely chop flags, except for a little garnish. Heat the rest of the sauce again. Remove the cucumbers from the oven and serve with sauce and meatballs.
Sprinkle with dill and garnish with egg and dill flags. Potato rösti taste good with it.