Cook bacon in large skillet on medium heat until crisp. Remove bacon. Set aside. Remove all but 2 tablespoons drippings from skillet. Add mushrooms and onion to skillet; cook and stir 6 to 8 minutes or until onion is tender and liquid has evaporated.
Arrange 1/2 of the bread slices in single layer in 13x9-inch baking dish. Top with spinach, mushroom mixture, bacon and Parmesan cheese. Layer with remaining bread slices.
Beat eggs in large bowl with wire whisk. Add milk, Italian seasoning, salt and pepper; mix well. Gradually pour into baking dish. Top with tomato slices. Sprinkle with Swiss cheese. Sprinkle lightly with additional Italian seasoning, if desired. Cover with plastic wrap. Refrigerate least 1 hour or overnight. (If refrigerated overnight, let strata stand at room temperature 1 hour before baking.)
Preheat oven to 325°F. Bake 55 to 60 minutes or until center of casserole is set and top is lightly browned. Let stand 10 minutes before serving.