For the potato tortilla, peel the potatoes and slice finely with a vegetable slicer. Chop the onion finely. Heat the oil in a non-stick frying pan and fry the onion and potatoes in it, turning frequently, until the potatoes are firm to the bite.
Clean the mushrooms, cut them into fine leaves and fry them briefly. Mix the egg with the milk in a bowl. Add the kitchen herbs and season everything well together with salt, pepper and nutmeg.
Flatten the potatoes and mushrooms in the frying pan, pour the egg mixture over them and let them fry at a low temperature. Using two plates, turn the tortilla to the other side and fry for another 1-2 minutes.
Arrange the potato tortilla and serve.