Peel the potatoes and cook them in the milk. Add enough water to fully cover the potatoes. Add the garlic cloves and salt. Cook until the potatoes are very tender. Drain with a slotted spoon, reserving the cooking liquid. Mash the potatoes with the garlic using a fork. If you have some, add a little chopped parsley. Add a ladle or two of the reserved milk to make the mashed potatoes creamy.

Meanwhile, finely chop the onion in a food processor and sauté it in a tablespoon of duck fat. Shred the pot-au-feu meat and chop it in the food processor as well. Add the tomato paste, beef stock concentrate, and water. Let simmer while you finish the mashed potatoes.

In a baking dish, spread the meat mixture, then cover with the mashed potatoes. Place under the broiler for 10 to 15 minutes. Serve with a mâche or endive salad.