Preheat oven to 375°F.
Grease baking dish lightly.
Melt butter in saucepan over medium heat.
Stir in flour to form a paste.
Slowly whisk in milk until smooth.
Add garlic, onion powder, salt, and pepper.
Cook sauce until thickened, about 5 minutes.
Stir in half of mozzarella cheese.
Add zucchini slices to the mixing bowl.
Pour sauce over zucchini and mix well.
Layer zucchini mixture in baking dish.
Sprinkle remaining mozzarella and Parmesan on top.
Optional: Add breadcrumbs for crunch.
Bake for 25-30 minutes until golden and bubbly.
Let rest 5 minutes before serving.