Soak the porcini mushrooms for about 30 minutes in 1⁄2 l warm water. Clean the mushrooms and wash if necessary. Clean, wash and cut the leek into large pieces.
Peel and chop 1 onion. Knead with minced meat, breadcrumbs, egg, mustard, salt and pepper. Form small balls from it. Heat the oil in a casserole. Brown the balls all around, take them out.
Fry the mushrooms in portions in the frying fat. Fry the leek briefly. Season with salt and pepper. Add meatballs again. Pour mushroom soaking water through a sieve lined with kitchen paper into the pot, bring to the boil.
Soaked mushrooms may be chopped a little smaller and added. Cover and stew for 15-20 minutes.
Wash parsley, shake dry and chop finely. Chop or slice the rest of the onions into fine rings. Heat 1 tbsp. butter in a large frying pan. Fry the onions in it until golden brown, remove and keep warm.
Heat 1-2 tablespoons butter in the frying fat and fry the spaetzle in 2 portions for about 5 minutes.
Carefully stir the crème fraîche and parsley into the mushroom-leek pot. Season to taste again. Add the spaetzle with fried onions.