Preheat oven to 400° F. Spray a casserole dish with nonstick spray. Set aside.
Combine Gruyère and Panko breadcrumbs in a bowl; set aside.
Scrub zucchini under cold water; cut off the two ends, but do not peel. Cut off two ends of the onion and peel. Grate the zucchini and onion in a food processor or box grater; toss with ½ teaspoon salt in a colander. Let sit for 30 minutes.
Squeeze liquid from zucchini and onion with your hands. Then, place in a double paper towel and continue squeezing as much liquid as possible. (See Tip 4)
Heat 3 tablespoons of the butter to foaming over medium heat in a 12-inch nonstick skillet. Add the grated zucchini and onion. Cook for 3 to 5 minutes, just until tender. Season with remaining ½ teaspoon of salt, pepper, and nutmeg. Add flour; stir well to combine and cook for a minute or two.
Add milk; stir constantly until thickened and bubbly. Correct the seasonings.
Transfer the zucchini mixture to the prepared casserole dish. Top with Gruyère/Panko mixture. Dot with the remaining tablespoon of butter. Can be made ahead of time up to this point. (See Tip 5)
Bake in a preheated 400ºF oven for 20 minutes or until bubbling hot and golden browned. Yield: 4 servings. (See Tip 6)