Simmer sliced mushrooms in butter until soft. Purée them with cream; season to taste with salt and freshly ground black pepper. Toast brioche slices. Spread each slice thickly with mushroom purée and place on a baking sheet. Top each “toast” with aslice of foie gras and spoon hot Cream Sauce, to which you have added an egg yolk, over each “toast”. Cover with freshly grated Parmesan and grill until golden.