Preheat the oven to 375°F (190°C ). Lightly grease a 9 x 13-inch baking dish and set aside.
Thinly slice the butternut squash into slices no thicker than 1/4 of an inch.
In a small saucepan, add the butter and place it over medium heat. Add the onion and garlic and cook until fragrant. Add the heavy cream, nutmeg, and cinnamon. Simmer for about 5 minutes or until it thickens slightly. Remove from the heat.
Add half the butternut squash slices into the baking dish, ensuring they are in an even layer. Season with salt and pepper. Pour half the cream sauce over the top. Add the remaining squash, followed by the remaining sauce. Cover with shredded Gruyere and Parmesan cheese. Cover the baking dish with foil.
Bake for 30 minutes, remove the foil, then bake for another 20 minutes.
Remove the casserole from the oven and let it sit for 10 minutes before serving.
Video
Notes
Topping: Add up to half a cup of either crushed Ritz crackers or panko after the foil has been removed.
TO STORE: Store leftover butternut squash casserole in an airtight container in the fridge. The casserole will be good for up to 5 days.
TO FREEZE: Place portions of the casserole in a shallow container and store it in the freezer for up to 6 months.