Wash zucchini, remove ends, and slice into 1/3 inch coins. Slice the onion very thinly. Peel garlic.
Beat eggs in shallow bowl.
In a separate shallow bowl, combine flour, onion powder, garlic powder, paprika, basil, oregano, seasoning salt, and bread crumbs. Stir to mix well.
Dip sliced zucchini into egg mixture, then into flour mixture.
Heat olive oil in skillet and add whole garlic cloves. As the garlic cloves become golden, crush them into the oil using a fork. Remove garlic before it becomes brown.
Place zucchini in hot oil. Don't overload the skillet as they should cook separated in the oil. Sprinkle a few slices of sliced onion over them. Turn zucchini over as they become golden brown and cook the other side.
Remove zucchini and onions to a paper towel to drain. Sprinkle with salt and pepper to taste (and more garlic powder and onion powder, if desired).
Arrange zucchini on the bottom of a greased 8x8 baking dish. Sprinkle a layer of Parmesan cheese and a layer of mozzarella, then with a layer of tomato sauce. (Note: if tomatoes are ripe and in season, sliced ripe tomatoes may be substituted for the sauce in this dish).
Continue to make layers of zucchini, cheeses, and sauce until all ingredients are used, ending with mozzarella.
Sprinkle with buttered bread crumbs, if desired.
Bake uncovered at 350F for 25 minutes or until bubbly. Let cool for a couple minutes before serving.