Preheat oven to 375 degrees Fahrenheit. Line a cookie sheet with parchment paper and set aside.

Cut spaghetti squash in half lengthwise and scoop out the seeds. Bake face-down on the cookie sheet for 35 minutes; add five more minutes for a larger squash and five less for a smaller one.

Remove from oven. Leave squash face down for five more minutes. After five minutes, flip over and allow to cool completely before stringing. These steps ensure that you will get crispy individual strands out of your squash.

To string, hold the squash over a plate or bowl and use a fork to scrape the flesh. The flesh will come apart into strands that look almost exactly like spaghetti.

Sauté the onion until golden.

Add frozen garlic, mushrooms and sauté. Once the mushrooms are cooked through, add spinach and allow to cook until slightly wilted. Transfer to a mixing bowl.

Add stringed spaghetti squash. Season vegetables with salt and pepper. Add eggs and cheese if using and mix well. Pour mixture into a nine- or 10-inch round pan for a larger casserole.

Bake at 375 degrees Fahrenheit for about 35 minutes or until very brown on top. Rewarm uncovered or enjoy at room temperature.