Preheat your oven to 375°F (190°C). Generously grease a 9x13-inch or 2.5-quart oval ceramic baking dish with softened butter.

In a medium bowl, whisk together the heavy cream, minced garlic, chopped sage, grated nutmeg, fine sea salt, and black pepper.

In a separate bowl, toss the shredded Gruyere and sharp white cheddar cheeses together.

Arrange one-third of the sliced butternut squash in neat, overlapping shingled rows across the bottom of the dish. Pour one-third of the garlic cream over the squash and sprinkle with one-third of the mixed cheese.

Repeat layering two more times with remaining squash, cream mixture, and cheese.

In a small bowl, mix panko breadcrumbs with melted butter. Scatter breadcrumbs and whole buttered sage leaves over the top layer of cheese.

Cover loosely with foil and bake at 375°F for 30 minutes. Remove foil and bake uncovered for an additional 15-20 minutes until squash is tender when pierced with a knife, cream is bubbling vigorously, and crust is golden-brown and crunchy.

Rest for 10 minutes before slicing into generous squares and serving warm.