Cut Zucchini (8) into 1/2-inch thick slices.
Lightly coat zucchini slices with Olive Oil (to taste) on one side, preferably the cut side. Season with Sea Salt and Pepper (to taste). Working in batches, grill zucchini on the rack of a covered grill over high heat on the coated side for 4-8 minutes, or until char marks just begin to form. Remove and set aside.
Preheat oven to 375 degrees F (190 degrees C). Butter a 13x9-inch rectangular baking dish.
To make the white sauce, in a medium saucepan melt Unsalted Butter (1/4 cup) and cook the Onion (1) over medium heat for about 5 minutes or until softened.
Add the Gluten-Free All-Purpose Flour (3 tablespoon) and cook 3 minutes or until light golden brown, stirring constantly.
Add the Whole Milk (2 cup), whisking to combine well. Bring to a boil. Reduce heat and simmer for 2 minutes, stirring often.
Stir in Fresh Basil (2 tablespoon), Sea Salt (1/2 teaspoon), Freshly Ground Black Pepper (1/4 teaspoon), and Ground Nutmeg (1/8 teaspoon). Remove from heat.
To assemble the casserole, start off by spreading a third, about 2/3 cup, of the sauce on the bottom of the prepared baking dish. Next, add a layer of zucchini strips with the grilled sides facing up. Then sprinkle half of the Shredded Mozzarella Cheese (4 ounce) and half of the Grated Parmesan Cheese (1 ounce) over the entire surface.
Repeat with a layer of sauce. It will be quite thick so just apply small dollops evenly over entire surface. Add zucchini strips, followed by the remaining Shredded Mozzarella Cheese (4 ounce) and Grated Parmesan Cheese (1 ounce). Finish it off with the rest of the white sauce then the remaining zucchini strips, placed so that the top layer features alternating grill marks.
Bake, uncovered, for 40 minutes or until bubbling and nicely browned.
Remove from the oven and let stand for 20 minutes.
Top with additional basil and Sea Salt Flakes (to taste) and serve.