Spread bread cubes in a single layer on an ungreased baking sheet. Bake in a 375 degree F oven for 10 to 15 minutes or until lightly toasted.
Meanwhile, in a large saucepan cook zucchini or yellow squash, sweet pepper, and onion in hot oil for 5 minutes. Stir in broth or stock and undrained tomatoes. Bring to boiling; reduce heat. Simmer, uncovered, about 5 minutes or until vegetables are just tender.
Ladle soup into bowls. Top each serving with toasted bread cubes and, if desired, Parmesan cheese.