Begin by peeling and cubing the butternut squash into 1-inch pieces. In a large pot, bring water to a boil and add the cubed squash. Cook for 10–15 minutes until tender but firm. Once cooked, drain the squash and set it aside to cool slightly.

In a medium skillet, melt 2 tablespoons of butter over medium heat. Add finely chopped onions and minced garlic, sautéing for about 5 minutes until the onions are soft and translucent.

In a large mixing bowl, lightly mash the cooked butternut squash with a fork to retain some texture. Stir in the sautéed onions and garlic, along with heavy cream, shredded cheddar cheese, salt, and pepper.

Preheat your oven to 375°F (190°C). Grease a baking dish with cooking spray or additional butter. Carefully transfer the butternut squash mixture into the prepared dish, spreading it evenly. Sprinkle a layer of panko breadcrumbs over the top.

Place the baking dish in the preheated oven and bake for 25–30 minutes. The casserole is done when the topping is golden brown and crisp.

Once baked, remove the casserole from the oven and let it cool for a few minutes. Garnish with freshly chopped herbs, such as parsley or thyme.