For the pesto, coarsely chop the walnut kernels, roast them briefly in a pan without fat, let them cool down a little and grind them finely in the universal chopper. Wash parsley and mint, dab dry, pluck off the leaves and chop finely.
Finely grate the cheese. Mix nuts, herbs and cheese and stir with the walnut oil. Season to taste with salt, put aside.
Peel and wash the potatoes and celery. Cut potatoes into cubes of about 3 cm, cut celery into cubes of about 1 cm. Boil up the vegetables in a pot with a little salted water, cover and stew for about 20 minutes.
Dab meat dry and rub with salt and pepper. Heat 1 tablespoon of olive oil, sear the meat on all sides and cook in the preheated oven (electric cooker: 125 °C/ convection oven: 100 °C/ gas: see manufacturer) for approx. 20 minutes.
In the meantime, clean, wash and pat dry the porcini. Cut the mushrooms into slices. Peel and chop onion. Heat the rest of the olive oil in a large pan and fry the mushrooms briefly.
Add the onion and fry over medium heat for about 5 minutes. Season with salt and pepper.
Take the meat out of the oven, cover and let it rest for about 5 minutes. Heat the milk. Place the potato-celery vegetables in a sieve, catching the dripping broth. Roughly mash the vegetables in a large bowl with a potato masher, gradually adding the hot milk and cooking liquid until the masher has the desired consistency.
Season to taste with salt, pepper and nutmeg.
Cut the meat into slices. Arrange mashed potatoes with meat and porcini mushrooms, drizzle with some pesto.