*Cream of mushroom may be substituted to make the dish vegetarian.
Preheat oven to 350 degrees F.
In a large sauce pan or saute pan, melt the 3 tbs butter over medium-low heat. Add onions and saute until onions are limp and turn translucent (they will look pearly).
Place onions in an ungreased 1.5-2 quart shallow baking pan.
In the same pot used to saute the onions, add the soup, soy sauce, water and evaporated milk, adding the milk slowly, while stirring to avoid lumps. Heat through. Add any additional salt & pepper to taste at this point.
Pour soup mixture over onions in baking dish. Add 1/3 of the shredded cheese; stir and spread onion mixture evenly over bottom of the pan.
Top with the sliced bread. Brush tops of bread with the 1 tbs of melted butter.
Bake at 350 degrees F for 15-25 minutes or until bread is a nicely toasted golden brown.
Slightly press toasted bread slices down in the sauce, but do not let them get covered in the sauce! Top casserole with remaining cheese and bake for an additional 5-10 minutes or until cheese is melted and is beginning to bubble.
Makes 6-8 side-dish servings.