Preheat the oven to 375°F. Slice the spaghetti squash in half lengthwise and scoop out the seeds and ribbing. Place the squash cut side down on a baking sheet and roast for 30 to 40 minutes.
In the meantime, heat a large nonstick skillet over medium-high heat. Lightly spray it with coconut oil. Add the onions and cook until translucent. Add the ground beef and cook until brown. Add the tomato sauce, stir, and let it simmer on low.
Once the spaghetti squash are cooked, set aside to cool down cut side up. When it is cool to the touch, use a fork to scrape and fluff the strands from the sides of the squash.
In a casserole baking dish, add the spaghetti squash at the bottom and press it down. Add the meat mixture on top and sprinkle it with cheese. Bake for about 20 minutes.
