Cut the spaghetti squash in half lengthwise. Be careful as it is tough to cut through. Place the squash on towels if necessary to keep it in place.
Remove the seeds by scraping the inside of each half with a spoon.
Drizzle the inside of each half with the vegetable oil and sprinkle with salt and pepper.
Place the squash cut side down on a baking sheet. Roast in the oven until tender when pierced with a knife, approximately 30-40 minutes. Remove from oven and let cool slightly to handle.
While the squash is cooking, cook the steak if not using leftover steak. Season with salt and pepper, and cook in a large skillet over medium heat just shy of your preferred doneness as it will cook a little more in the oven.
Also while the squash is cooking, make the alfredo sauce. In a medium saucepot, combine the butter and heavy cream and heat over medium heat. When the butter is melted, add the nutmeg and stir to combine.
Remove the sauce from the heat. Add the Parmesan cheese and stir until melted. Season to taste with salt and pepper (cheese salinity varies greatly, so you might need no salt, or quite a bit of salt). Set aside.
Fluff up the interior of each squash by using a fork and shredding at the squash flesh. This will make the "noodles"!
Slice the steak. Add the steak, spinach, sundried tomatoes, gorgonzola crumbles, and 1/2 cup sauce to each of the halves of squash. Stir with a fork to combine. Add more of any ingredient as desired.
Sprinkle the mozzarella cheese across the top of each squash boat. Bake for approximately 15 minutes until the cheese is melted. If you want browned cheese, you can pop the squash under the broiler once the cheese is melted (just make sure it is no longer on parchment or silicone or anything else that would burn!).
Drizzle with additional sauce (if you have some and if desired) or additional mix ins like sundried tomatoes and serve immediately.
Any cut of steak here will work, so use your favorite. You can also substitute for leftover steak that is already cooked (approximately 3/4 pound cooked for same yield).The alfredo sauce is best fresh, but all other components can be made in advance.