Preheat oven to 350 degrees. Steam vegetables until just tender. Set aside to cool. Fine chop steamed vegetables and mix with egg white, salt, pepper, mushrooms and bread crumbs. Form mixture into patties.
Place patties, topped with onion slices, on a lightly oiled baking sheet and bake, turning once, until brown and crispy on both sides, about 45 minutes.
Per serving: 111 cal; 12 g prot; 97 g sod; 22 g carb; 4 g fat; 0 chol; 101 mg calcum