Wash, peel and cut the potatoes into pieces. Peel garlic and cut into slices. Cook the potatoes and half of the garlic in little boiling salted water for about 20 minutes.
In the meantime, clean and wash the rocket and let it drain well. Wash the tomato, dab dry and cut into slices. Peel and chop the shallot. Mix vinegar, salt, pepper and diced shallots. Finally, fold in the oil.
Wash the tuna, dab dry, sprinkle with lemon juice and season with salt and pepper. Heat 10 g fat in a pan and fry the steak on each side for about 3 minutes. Finally, fry the remaining slices of garlic and lemon.
Drain potatoes, add milk and remaining fat, heat up again and mash coarsely. Season to taste with salt, pepper and nutmeg. Arrange tuna steak, mashed potatoes and salad on a plate.
Pour vinaigrette over the salad. Add the roasted garlic to the puree and serve immediately.