Cook and stir sausage in large skillet on medium heat 5 minutes or until lightly browned. Add mushrooms and onion; cook and stir 5 minutes or until onion is tender. Spread 1/2 of the sausage mixture in bottom of 13x9-inch baking dish. Top with bread slices in a single layer, overlapping as necessary. Top with remaining sausage mixture.
Beat eggs in large bowl with wire whisk. Add milk, thyme, garlic powder and sage; mix well. Gradually pour into baking dish. Cover with plastic wrap. Refrigerate 1 to 4 hours.
Preheat oven to 450°F. Bake 20 to 25 minutes or until bread is golden brown. Let stand 5 minutes before serving.