Preheat the oven to 300 F. Saute mushrooms in butter, set aside. Separate the egg yolk and place it in the middle of a buttered ramekin. Beat the egg white with a whisk until it starts to foam and doubles in size. Add seasoning and chopped herbs to the egg white, gently folding them in. Pour the white on top of the yolk, making sure the yolk stays in the middle. Bake the egg in the oven for 8 minutes and broil for 1-2 minutes. Meanwhile reheat the mushroom, melt the cheese on top of the bread under a broiler and once the egg is out of the ramekin, assemble the "omelette"