Heat the olive oil in a saucepan and fry the mushrooms in it. Swirl the pot a little from time to time so that the mushrooms are well covered with the olive oil. Then melt a bit of butter over it. Deglaze with milk and cream. Add the grained vegetable stock, pepper, salt, nutmeg and the herbs and stir well. Simmer on a low flame for about 30 minutes. Thicken with flour if necessary.