1Grease rectangular pan, 13x9x2 inches. Cook sausage in 12-inch skillet over medium heat 7 to 8 minutes, stirring occasionally, until no longer pink; drain. Melt butter in same skillet over medium-high heat. Cook spinach, zucchini, bell peppers and onion in butter about 5 minutes, stirring frequently, until zucchini is crisp-tender.
2Break each bread slice into 4 pieces. Layer sausage, vegetables and bread in pan. Beat eggs, milk, mustard, salt and pepper with hand beater or wire whisk until blended; pour over bread. Sprinkle cheeses over top. Cover and refrigerate at least 4 hours but no longer than 24 hours.
3Heat oven to 325ºF. Cover and bake 30 minutes. Uncover and bake about 45 minutes longer or until top is golden brown and knife inserted in center comes out clean.