Arrange torn up clean spinach leaves in bottom of medium size glass bowl. Put shredded cheese or cubed cheese on top. Spread mushrooms over cheese. Cover with sliced eggs.
In small bowl cream together well, mayonnaise, sour cream, milk, lemon juice and sugar. Stir in onions. Spread over top of salad right to the sides of bowl. Cover. Chill overnight. To serve, remove cover, sprinkle bacon over top.
ROMAINE MAKE AHEAD: omit spinach and use Romaine lettuce in above salad. Every bit as good.
VARIATION: Omit mayonnaise and sour cream. Add 8 oz. cream cheese mixed with 1/2 cup sour cream.