In heavy skillet melt butter and sauté mushrooms for about 5 minutes. Add cooked chicken and stir to distribute. Add sherry, stir and cook 5 minutes. Add supreme sauce, still well and add heavy cream. Simmer slowly for 5 minutes, stirring occasionally so sauce does not scorch. Taste for season and add salt (if necessary) and white pepper.
In an 8 cup buttered casserole, add cooked spaghetti. Pour hot chicken and sauce over spaghetti and top with ¾ cup hollandaise. Sprinkle with Parmesan cheese. Brown lightly under broiler.
SUPREME SAUCE:
In blender, add butter, flour, salt, pepper, ½ cup heavy cream and hot chicken stock. Cover container and turn motor on low speed. When blades have reached full momentum, switch motor to high and blend for 30 seconds.