Wash the potatoes, peel them, cut them into small pieces and cook them in a saucepan in salted water.
Finely dice the onion, cut the garlic into thin slices. Clean the mushrooms and cut into small pieces.
Heat a large non-stick pan and fry the mushrooms over a high heat until brown. Add the onion cubes and garlic slices and fry briefly. Add the tomato paste and roast. Season with salt and pepper.
Deglaze the mushroom pan with the port wine and let the liquid boil down almost completely. Then pour in the broth and let it boil down to the consistency you want. If necessary, add a little more water. Finally season with soy sauce and pepper.
Drain the soft-cooked potatoes and let them evaporate briefly in the open pot on the switched-off stove. Add the hot milk and mash the potatoes. Season with salt, pepper and nutmeg.
Arrange the mashed potatoes on a plate, drape the mushroom sauce over it, garnish with parsley leaves and serve.