Preheat oven to 350 degrees.
Cut spaghetti squash in half and scrape out the inside. (pop it in the microwave first as it makes it easier to cut) Cover inside of the spaghetti squash with olive oil and sprinkle salt and pepper. On a cookie sheet covered with parchment paper, turn the spaghetti squash cut side down and roast for 20-35 min depending on the size of the squash until the inside is al dente when scrapped.
Meanwhile, brown ground beef with onion, then drain. Add spaghetti sauce and simmer on low for 15 min. If it gets too high, you can add a bit of beef broth.
Scrap the spaghetti squash with a fork, then place all of it inside several paper towels to squeeze out moisture. I did this twice with two sets of fresh paper towels so it wouldn't be watery.
Grease a 9 x 9 square baking pan. Add spaghetti squash to the bottom of the casserole dish, top with spaghetti sauce, mozzarella cheese, and parm cheese.
Baker in a 350 degrees oven until cheese melts for approx 5-10 min then turn oven to broil for 1-2 min and watch carefully as mozzarella cheese starts to bubble and brown a little.
Top with parsley for pics to give some dimension for color. I added a few pics to document for ideas. If you have a larger spaghetti squash, you may not need to add all of it to the casserole dish.