Scrape out the "noodles" into a large bowl using a fork and set aside.
Meanwhile, heat the olive oil in a large skillet over medium heat.
Add the onion, season with salt and pepper, and cook for about 5 minutes, or until onion starts to soften and become translucent.
Add the garlic and cook for another minute or two.
Add the ground been to the skillet, and cook until cooked through and browned, crumbling it as it cooks.
Once the ground beef is cooked through, drain any excess grease from the pan.
Add the oregano, cinnamon, and allspice, cook for a minute or two, then add the crushed tomatoes, and simmer until heater through.
Meanwhile, prepare the Greek Yogurt Bechamel according to the instructions below.
Preheat your oven to 400°F, and coat a 2 quart casserole dish with oil or cooking spray.
Pour the tomato meat sauce over the spaghetti squash. Depending on the size of your spaghetti squash, you may not need all of it, or you may also wish to keep some set aside to pour additional sauce over when serving.
Transfer the spaghetti squash and tomato meat sauce mixture to the casserole dish.
Spread the bechamel evenly over the spaghetti squash mixture, then sprinkle with the parmesan cheese.
Bake for 40-50 minutes, or until bubbling, and the bechamel is set and browning.
For the Greek Yogurt Bechamel:
Stir together all of the ingredients in a bowl, and set aside until assembling the pastitsio in the casserole dish.
Greek Yogurt Bechamel adapted from Sumptuous Spoonfuls Moussaka.