Preheat oven to 400°F.

Melt 1 tablespoon butter in a skillet over medium heat. Add onion, garlic, 1 tsp salt, and 1/4 tsp pepper. Cook until onion is translucent, about 5 minutes. Remove from heat.

In a mixing bowl, combine the remaining melted butter, heavy cream, egg yolk, and the cooked onion mixture. Stir well.

Fold in zucchini slices, oregano, basil, and 1/4 cup of the Parmesan cheese. Mix until evenly coated.

Spray a baking dish with cooking spray. Spread zucchini mixture in the dish, stacking slices horizontally. Distribute any leftover mixture evenly.

Cover dish with foil and bake for 10-15 minutes until zucchini softens.

In a small bowl, mix panko breadcrumbs with remaining Parmesan and any leftover herbs.

Remove foil from dish. Sprinkle the breadcrumb mixture evenly over the top.

Return to oven and bake for another 15 minutes, or until top is golden brown and filling is bubbling.

Remove from oven, sprinkle with salt and pepper, garnish with fresh basil, and serve immediately.