Clean the oyster mushrooms and chop half of them into small pieces. Fry the chopped mushrooms in butter with the sliced shallots, season generously with salt and pepper and leave to cool. Make a slit down the length of each fillet and open. Stuff the fillets with the mushroom mixture. Fasten tightly with toothpicks. Fry in butter, season with salt and pepper and remove from the pan. Leave aside to cool. The sauce will be prepared in the same pan. Roll out the pastry and wrap it around the fillets, coat with egg yolk and cook in a preheated oven at 350°F for 20 minutes. Deglaze the pan with the brown Kasteelbier and stock. Add some marjoram and lemon thyme. Cook until the sauce has reduced by a third, sieve and add a knob of butter. To prepare the Flemish style potato puree : fry the bacon in a little butter with sliced shallots and cultivated mushrooms, season with pepper, add two crushed cloves of garlic. Cook the floury potatoes and then strain away half the water, mix the mushrooms and bacon with the potatoes and the finely chopped parsley. Fry the whole oyster mushrooms in walnut oil as a garnish. Line a ring mould with blanched green cabbage and fill with the puree. Close carefully and place the puree in the middle of the dish, top with sauce, surround with the oyster mushrooms and sliced pork fillets. Garnish with watercress sprigs.