Peel and thinly slice the potatoes and kohlrabi, layer in the prepared dish.
Whisk together the single cream, milk and cheese. Remove the thyme leaves, add to the sauce, press in the garlic, season. Pour the sauce over the vegetables.
Bake for approx. 45 mins. in the centre of an oven preheated to 180°C. Remove the gratin from the oven, turn the heat up to 200°C.
Heat the clarified butter in a frying pan. Brown the meat in batches for approx. 1 min. on each side, remove and season.
Finely chop the parsley. Mix the butter with the parsley, nuts and breadcrumbs.
Spread the mixture over the medallions and press down gently.
Place the medallions on a baking tray lined with baking paper, place the gratin on the tray alongside the meat. Cook together for approx. 15 mins.