Remove the dark green of leeks, cut lengthwise, wash and cut into short pieces.
Simmer the pieces of leek in the soup for 10 minutes. Peel potatoes and cut into small cubes. Add them to the leek and cook 10 minutes more. Add bay leaf and season the vegetables with salt, pepper and nutmeg.
Preheat oven to 180 ° C.
Peel onion and chop finely. Seasoning minced meat with salt, pepper and onion. Heat oil in a pan and fry minced crumbly.
Drain vegetables and add soup to the minced meat. Simmer briefly.
Fill the diced vegetables in a baking dish. Cover with minced and sauce. Gratinate casserole in the oven for about 30 minutes.