Soak the porcini mushrooms in the boiling water. Set aside.
Coat each piece of chicken in the flour, seasoned with salt and pepper.
Melt 50 grams of the butter in a large skillet and fry the chicken in batches on both sides until just browned. Transfer to a slow cooker and add the porcini mushrooms and liquid.
Meanwhile, place the remaining butter, onion and garlic into the Thermomix bowl and chop 3 sec/speed 6. Scrape down sides of bowl and sauté 7 min/Varoma/speed 1.
Place mushrooms, celery, carrots and potatoes into the slow cooker. Tip the contents of the Thermomix bowl on top. Place chicken stock, mustard and nutmeg into the Thermomix bowl, combine 5 sec/speed 8. Pour into the slow cooker. Cook on high for 4 hours. Taste and adjust seasoning as needed.
Serve with egg noodles, mash, polenta or on its own.