Heat oven to 400? F.

In a large bowl, add squash cubes and coat thoroughly with olive oil and season with salt and pepper. Place squash in a single layer on a baking sheet, and roast for 20-25 minutes, until tender. Remove from the oven and set aside.

Heat 2 tablespoons butter in a large skillet over medium-high heat and saut? onion for 8 minutes, until softened. Add garlic and cook for 1 minute. Season with salt and pepper.

Lower heat to medium and add cooked spinach to onions and garlic. Stir in heavy cream 1 tablespoon at a time, until fully incorporated. Add nutmeg and stir. Continue cooking for 5 minutes, or until thickened. Add roasted butternut squash to pan and stir gently to combine.

Transfer mixture to a 9x13-inch baking dish and sprinkle with gruyere and parmesan cheese. Mix gently to incorporate.

In a medium bowl, mix together 3 tablespoons melted butter and breadcrumbs evenly over the top.

Place in the oven and bake for 30-35 minutes, or until bubbly and cheese is melted. Once done, let cool for 5 minutes before serving.