French Onion Casserole

Preparation : Cut onions crosswise into 1/4-inch slices; cut each of the slices in half. Melt butter in a large skillet over medium-high heat; add onion and mushrooms and cook, stirring constantly, until tender. Spoon mixture into a lightly greased 2-quart baking dish. Sprinkle with 1 cup cheese. Combine soup, milk, and soy sauce; pour over cheese. Top with bread slices, and sprinkle with remaining 1 cup cheese and parsley. Cover and chill 4 to 8 hours. Remove baking dish from refrigerator, and let stand at room temperature 30 minutes. Cover and bake at 375 degrees for 30 minutes. Uncover and bake 15 to 20 additional minutes or until thoroughly heated. Let stand 5 minutes before serving.