Preheat your oven to 375°F (190°C). Grease a 9x13-inch baking dish and set aside.

In a large skillet, melt the butter over medium heat. Add the chopped onion and minced garlic, and sauté until the onion becomes translucent and fragrant.

Add the cubed butternut squash to the skillet and cook for about 10 minutes, or until the squash is tender. Stir occasionally to prevent sticking.

Sprinkle the flour over the cooked squash and onion mixture. Stir well to coat the vegetables evenly.

Slowly pour in the milk and broth, stirring constantly to avoid lumps. Cook for an additional 2-3 minutes, or until the sauce thickens.

Season the mixture with nutmeg, salt, and black pepper. Stir in the grated Parmesan cheese until it melts and combines with the sauce.

Transfer the creamy butternut squash mixture to the greased baking dish, spreading it evenly.

In a small bowl, combine the panko breadcrumbs, chopped parsley, and olive oil. Mix well until the breadcrumbs are coated with the oil.

Sprinkle the breadcrumb mixture over the top of the casserole, creating an even layer.

Bake the casserole in the preheated oven for 25-30 minutes, or until the top is golden brown and the casserole is bubbling around the edges.

Remove from the oven and let it cool for a few minutes before serving. Garnish with additional chopped parsley, if desired.