Preheat your oven to 375°F and grease a 9x13-inch baking dish with butter. Peel and cube your butternut squash into bite-sized pieces, removing all seeds and stringy bits carefully.

Melt butter in a large skillet over medium heat, then sauté diced onion until translucent, about 4 minutes. Add minced garlic and cook an additional 30 seconds until fragrant and golden brown.

Toss butternut squash pieces into the skillet and stir constantly for 3 minutes to coat with buttery aromatics. Pour in vegetable broth and reduce heat to medium-low, simmering for 12 minutes until squash begins softening.

Remove from heat and stir in heavy cream, Parmesan cheese, sage, thyme, nutmeg, salt, and pepper thoroughly. In a separate bowl, whisk panko breadcrumbs, melted butter, pecans, and beaten eggs until clumpy and combined.

Transfer the butternut squash mixture to your prepared baking dish, spreading evenly across the bottom. Sprinkle the pecan breadcrumb topping generously across the surface, pressing slightly to distribute evenly.

Bake for 40 minutes until the topping turns golden brown and the squash casserole crowd-friendly filling bubbles slightly at the edges. Let rest for 5 minutes before serving to allow flavors to settle.