Coat an 8x12” or 9x9-inch glass baking dish with butter or spray with cooking spray.
Slice zucchini and tomatoes in 1/8-inch slices and place on paper towels.
Sprinkle with salt.
Allow vegetables to drain on paper towels for two to three hours if possible.
In a mixing bowl, measure the cheddar cheese, ½ cup parmesan cheese, basil, garlic, oregano, salt and pepper.
Stir really well to combine seasonings with cheeses.
Set aside.
Spread half of the drained zucchini into bottom of prepared baking dish.
Sprinkle ¼ of the cheese mixture on top of the zucchini.
Spread half of the drained tomato slices over top of the zucchini.
Sprinkle another ¼ of cheese mixture over top of the tomatoes.
Repeat layering the remaining zucchini, sprinkle with another ¼ of the cheese mixture.
Then layer the remaining tomato slices.
Sprinkle the final 1/4 of the cheese mixture over top.
Sprinkle with the remaining ½ cup Parmesan cheese.
Meanwhile, in a small skillet over medium heat, melt butter.
Add onions and sauté until almost transparent—just a few minutes.
Mix in three quarters of a cup of Panko Italian-style bread crumbs and sauté until all the butter is immersed—a minute or two.
Sprinkle the onion and Panko bread crumb mixture over the zucchini and tomatoes.
Lightly cover the casserole dish with aluminum foil.
Bake at 375° for 45 minutes.
Remove foil and bake for five minutes more to brown casserole slightly.