Brown sausage in skillet; drain and cool.
Beat eggs, milk, salt, and dry mustard.
Trim crust from sides of bread; tear up into small pieces. Add to egg mixture.
Stir very well – soaking bread.
Add cheeses, mushrooms, and sausage. Stir.
Pour into greased (Pam) 9x13 baking dish.
Bake at 350° for 50-55 minutes – or until edges are slightly brown.
Note: I often just sprinkle the mushrooms over the top of 1/2 or 2/3rds of the casserole, as not everyone likes mushrooms.