Preheat the oven the day before (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer). Wash meat, pat dry and season well with salt and pepper. Place on a fat pan (deep baking tray) of the oven and roast in the hot oven for 2 1⁄2-3 hours.
Take it out and let it rest a little on a cake rack or oven rack.
Meanwhile clean the mushrooms, possibly wash and slice them. Heat the oil in a large pan. Sauté the mushrooms briefly in the oil in portions. Season with salt and pepper and remove.
Cut the rind off the camembert thinly, dice the inside roughly. 1⁄4 l Bring water to the boil, melt camembert and processed cheese in it while stirring. Stir in cream, bring to the boil again. Season with salt and pepper.
Take it off the stove.
Cut the roast into approx. 1.5 cm thick slices and layer them like roof tiles in a large ovenproof dish. Spread the mushrooms on top and pour the sauce evenly over them. Cover and leave to soak overnight in the refrigerator.
Preheat oven the next day (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer). Bake the roast for 35-40 minutes in a hot oven. Wash parsley, shake dry and chop finely. Remove roast from the oven and sprinkle with parsley.
Baguette or farmhouse bread tastes good with it.