Brown the butter in a sauté pan. Sauté the mushrooms, nuts and parsley (setting aside some of the parsley for garnish) over a high heat, stirring continuously, for about 4 min. Add the lemon zest, nutmeg, salt and pepper. Coarsely chop the mixture in a food processor at max. speed for about 5 sec. Season the mushroom pesto to taste.
Use the pesto in freshly cooked pasta. Stir 200 ml boiling pasta water into the pesto and stir it into the drained pasta. Serve with 100 g grated cheese.
The mushroom pesto can be stored in the refrigerator for 1-2 days.