Preheat oven to 375 degrees Fahrenheit (190 degrees Celsius).

Cut spaghetti squash in half and remove seeds. Place squash halves face down on a baking sheet lined with parchment paper. Bake for 35 minutes. Remove from oven and let sit five minutes, then flip over and allow to cool completely. (For a larger squash, you may need five to 10 more minutes of baking time.)

Use a fork to create spaghetti-like strands from the flesh of the squash. Set the empty shells aside for a cool serving option.

While squash is cooking, prepare the rest of the dish: In a large frying pan, melt butter. Sauté onion until it begins to soften, then add garlic and mushrooms. Season with salt. Sauté vegetables until soft. Add spinach leaves and heat just to wilt. Add crushed tomatoes and mix to combine.

Lower oven heat to 350 degrees Fahrenheit (175 degrees Celsius).

Combine vegetable mixture with the prepared spaghetti squash. Toss with four ounces feta cheese. Divide the mixture between the empty squash shells or pour it into a casserole dish. Top with remaining two ounces feta cheese.

Bake just until the cheese begins to melt. Top with fresh basil and serve.