Cook sausage and onion in medium skillet over medium heat, stirring occasionally, until sausage is no longer pink; drain well.
Arrange bread slices in ungreased 11x7 pan, cutting to fit if necessary. Spread sausage mixture over bread. Top with bell peppers and zucchini.
Beat eggs and ricotta cheese in medium bowl, using fork, until blended. Stir in milk.
Pour over mixture in baking dish. Sprinkle with Parmesan cheese. Cover tightly and refrigerate at least 2 hours but no longer than 24 hours.
About 1 hour before serving, remove strata from refrigerator.
Preheat oven to 350 degrees F.
Uncover and bake 45-55 minutes or until top is puffed and center is set.