Preheat oven to 400 degrees.
Cut spaghetti squash in half, lengthwise, and discard all of the seeds. Place halves face down (skin side up) in a glass baking dish, and bake for 45 minutes until out skin feels soft.
Scoop out the cooked squash and transfer into a bowl, squeezing out any excess water. Mix in the egg, spices, and salt. Place parchment paper onto a baking sheet and spread the squash mixture out flat into a rectangular crust.
Bake for 15-20 minutes until the crust becomes crispy.
Take the squash crust out of the oven, and top with feta, mushrooms, spinach, tomato slices, and then mozzarella.
Bake for another 10 minutes on 500 degrees until the cheese becomes golden brown. Serve immediately.