Peel, wash and cut the potatoes into pieces. Cover and cook in salted water for about 20 minutes.
Onions peel and chop finely. Wash and halve the tomatoes. Wash lamb's lettuce thoroughly, spin dry and clean.
Dab the meat dry and cut it into about 8 medallions. Heat the oil in a large pan. Fry the meat for 2-3 minutes on each side. Season with salt and pepper. Wrap in aluminium foil and keep warm.
Steam onion in frying fat until transparent. Add tomatoes and fry briefly. Season with salt, pepper and thyme and braise covered over medium heat for about 3 minutes. Season to taste.
Heat milk and butter in a pot. Drain the potatoes and mash them with a potato masher, adding the milk. Season to taste with salt and nutmeg. Carefully fold in lamb's lettuce. Arrange lamb's lettuce mashed potatoes with medallions and braised tomatoes.
Drink tip: white wine, e.g. a Pinot Gris.