Preheat your oven to 375°F and lightly coat a medium casserole dish with non-stick spray.
Melt the butter in a large saucepan over medium heat. Toss in the minced garlic and cook until it’s fragrant, about 1 minute. Sprinkle in the flour, whisking constantly, and cook for another minute until it turns into a roux.
Gradually add the milk, stirring constantly. Let it simmer until it thickens, which should take about 2 minutes.
Turn off the heat and mix in 1/2 cup of Gruyere, 1/2 cup of mozzarella, and 1/4 cup of Parmesan. Stir until the cheeses are fully melted and the sauce is smooth. Season with salt and pepper.
Place a layer of zucchini slices at the bottom of the casserole dish. Pour about 1/3 of the cheese sauce over the zucchini. Continue layering zucchini and cheese sauce until all the zucchini is used up.
Top the final layer with the remaining cheeses. Bake in the oven for 20
to 25 minutes, or until the top is golden brown and bubbly. Garnish with fresh parsley before serving. Enjoy!