Peel and wash the potato, then cut it into pieces and boil until cooked.
Wash the mushrooms and cut them into small pieces. Brown them in a frying pan with the butter, stirring them very little to prevent them from releasing too much water.
At the same time in a casserole, pour the oil, add the pressed garlic, the minced shallots, a pinch of grated nutmeg and heat over medium heat until the shallots are translucent.Pour in the white wine and bring to a boil. Add the white leek in slices, the celery branch in small pieces, the mushrooms, the vegetable broth and the sage leaf.Reduce heat and simmer covered for 5-10 minutes.
Remove the sage leaf, add the potato pieces, pour the cream into the casserole, mix, then mix everything using your immersion blender.
Serve the soup hot in bowls and season to taste. Readjust the cream if necessary.You can serve with some pan-fried mushrooms, croutons, toasted pine nuts or some chopped fresh herbs.
Notes
Never pepper a soup for cooking, it will make it so hot you won't be able to eat it.