Peel and wash the potatoes and cook them in salted water for about 20 minutes. Clean and wash broccoli and cut into small florets. Add to the potatoes 5 minutes before the end of the cooking time and cook.
For the sauce, cut the bacon into cubes. Peel and finely dice the onion. Clean and dab dry the mushrooms and carefully unscrew the stems. Cut the stems into small cubes. Leave the bacon in a pot until crispy.
Briefly sauté onion and mushroom stems and dust with flour. Deglaze with cream and milk, stir in broth and bring to the boil. Season to taste with pepper and a little salt. Drain potatoes and broccoli well and mash finely.
Add fat and stir in. Season with salt, pepper and nutmeg. Pour into a piping bag with star-shaped spout and squirt the mixture into the mushrooms. Pour the bacon sauce into one large or 2 smaller ovenproof dishes.
Wash the parsley, cut into strips and sprinkle over the sauce. Add the mushrooms. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for about 15 minutes.