Wash the potatoes, peel and cut into approx. 2 cm cubes, then cook them in salted water until they are firm to the bite, drain and allow to evaporate.
In the meantime, wash and clean the leek and cut into rings. Clean the mushrooms and cut into quarters, depending on their size, about the size of the potatoes. Peel and dice the onions. Preheat the oven to 180 degrees circulating air.
Put 2 tablespoon oil in a high pan and heat. Fry the onion, mushrooms and leek in it. Add the thyme chopped or ground and season the vegetables with salt and pepper. Pour in the broth and cream and simmer for 3 minutes.
Mix the starch in a little cold water according to the package instructions and use it to bind the sauce. Add the potatoes, mix in and pour into a baking dish. Attention: The baking dish should have a capacity of at least 4 liters.
Cut the fillet into 8 small medallions, flatten them lightly and season with salt and pepper. Heat the remaining oil in a pan and fry the medallions on both sides. Then distribute the meat on the vegetables and sprinkle with the grated cheese.
Bake the pork fillet casserole in the oven for about 20 minutes.