Wash the potatoes, cut them into slices and place them on a baking tray lined with baking paper. Sprinkle evenly with 1 tablespoon of oil and season with salt and pepper. Cook in the preheated oven (electric cooker: 200 °C/ circulating air: 175 °C/ gas: level 3) for 15-20 minutes.
Wash the thyme, shake dry and, except for something to garnish, pluck leaves from the stalks. Peel the onion and cut into slices. Clean and trim the mushrooms and halve them according to size. Wash and quarter the tomatoes.
Wash the meat, dab dry and season with salt and pepper. Heat 1 tablespoon of oil in a large pan. Fry the meat for approx. 4 minutes, turning it over. Remove the meat, add the onion, mushrooms and tomatoes to the hot frying fat.
Fry over reduced heat for about 2 minutes, turning and season with thyme, salt and pepper. Add the milk, bring to the boil and dissolve the gravy. Return the meat and turn in the sauce. Arrange potatoes, meat and sauce on plates.
Garnish with thyme.