Zucchini Gratin Casserole

6 tablespoons butter

1 onion, chopped

2 pounds zucchini, cut into 1/4-inch slices

2 teaspoons kosher salt

1 teaspoon freshly ground black pepper

1 teaspoon chopped fresh parsley

2 tablespoons all-purpose flour

1 cup hot milk

1 tablespoon chicken bouillon granules

1 cup shredded extra-sharp Cheddar cheese

4 slices bread, crumbled

½ cup melted butter

Preheat oven to 400 degrees F (200 degrees C). Grease an 8x10-inch baking dish.

Melt 6 tablespoons of butter in a large skillet over medium heat, and cook and stir the onion until soft, about 6 minutes. Mix in the zucchini, and cover the skillet; cook until tender, about 10 minutes. Stir in the kosher salt, black pepper, and parsley, and cook uncovered for 5 more minutes. Stir in flour, and reduce heat to a simmer.

Mix the hot milk and chicken bouillon granules together in a small bowl, stirring until the bouillon has dissolved. Pour the milk into the skillet, and bring to a simmer, stirring constantly until the milk thickens into a sauce, about 5 minutes. Pour the zucchini mixture into the prepared baking dish. Sprinkle with Cheddar cheese. Mix the bread crumbs with 1/2 cup of melted butter, and sprinkle the crumbs over the casserole.

Bake in the preheated oven until the casserole is bubbling and the top has browned, about 20 minutes.