Preheat oven to 350.
Over medium heat, saute the zucchini rounds on a griddle, grill pan, or in a skillet until they are browned and slightly crisp on the outside.
Spread some spaghetti sauce into a 9x13 baking dish. You need enough to coat the bottom generously to prevent your zucchini from sticking.
Dip each zucchini round into the egg whites and then into the bread crumbs, once per side. Lay breaded zucchini rounds on top of the spaghetti sauce in the pan.
Preheat a couple of tablespoons of olive oil in a medium skillet. Saute onions until they begin to brown. Add garlic and saute until onions are translucent.
Spread the onion mixture over the zucchini, then top with more spaghetti sauce and cheese.
Bake for 30 minutes, until cheese is melted and bubbly.
While baking, prepare spaghetti according to package directions.
Sprinkle the zucchini and spaghetti with a bit more Parmesan cheese and serve.