Preheat oven to 325. Arrange bread in 15-1/2x10-1/2" jely-rolly pan. Bake 20 minutes or until crisp. Cool. Meanwhile, brown sausage in large skillet over medium-high heat; drain on paper towels. Add onions and thyme to skillet; cook until onion is translucent, about 2 minutes. Remove from heat. Stir in sausage and spinach to combine; cool completely. Arrange bread in shallow 3-quart baking dish. Sprinkle 1 cup cheese over top, then layer with sausage mixture and remaining cheese. Beat eggs, milk, mustard, in bowl until well blended. Carefully pour into prepared dish; sprinkle top with tomato. (Can be made ahead. Cover and refrigerate up to 24 hours.) Bake 1 hour or until center is just set. Let stand 10 minutes before serving. (wrv) until center is just set.