Clean and wash soup greens. Dice the celery, slice the carrots and cut the leek into rings.
Melt 20 g of fat in a saucepan, add the soup greens and fry briefly while stirring. Season with salt and pepper. Deglaze with 1/4 l water and cook for another 3-4 minutes. Pour onto a sieve and drain.
Peel onion and chop finely. Squeeze out the rolls and knead together with the mince, egg, onion, mustard, salt and pepper to a smooth mass. Mix approx. 2/3 of the soup greens into the mince mixture and form a loaf with moistened hands. Place in a roasting pan and spread the rest of the soup greens all around. Add 4-5 tablespoons of water and fry in the preheated oven (top / bottom heat 200 ° C or fan oven 180 ° C) for approx. 45 minutes. If necessary, add a little water in between.
Wash the potatoes and cook in salted boiling water for 20 minutes. Melt the remaining fat (30 g) in a saucepan and sweat the flour until golden. Pour in the stock and milk while stirring. Bring to the boil and simmer for approx. 8 minutes. Season the bechamel sauce with salt, pepper and nutmeg. Finely chop the parsley and add to the sauce.
Take out the meatloaf and let rest for 5-10 minutes.
In the meantime, drain, rinse and peel the potatoes. Halve depending on the size and add to the hot sauce.
Cut the meatloaf and arrange on a platter with the braised vegetables and serve with the bechamel potatoes.