Peel and slice onions crosswise. cut the slices into halves. Melt the butter in a skillet over medium high. Add onions and mushrooms. Cook until tender. Stir constantly. Spoon mixture into a greased 2 quart dish. Sprinkle top with 1 cup cheese. Mix soup, evaporated milk and soy sauce. Pour over cheese. Put bread slices over the layers and sprinkle with the remaining 1 cup cheese and parsley. Refrigerate casserole, covered, for 4 to 8 hours. Let stand at room temperature for 30 minutes before baking. Bake at 350 for 15 to 20 minutes. Let stand for 5 minutes before serving.