Place bread slices, slightly overlapping, in bottom of lightly greased 9 x 9-inch baking dish. In large bowl, thoroughly combine eggs, milk, bacon, 2 Tbsp Swiss cheese, 2 Tbsp cheddar cheese, mushrooms, and salt. Pour mixture over bread slices. Sprinkle potatoes over egg mixture and top with remaining cheese. Cover and refrigerate overnight.

 

Uncover, then bake at 350 degrees F until lightly browned and knife inserted in center comes out clean, about 40 to 45 minutes.