Preheat oven to 375 degrees F. Line a baking sheet (or oven-safe baking dish) with parchment paper or grease with olive oil or cooking spray. Cut each spaghetti squash in half lengthwise, scoop out the seeds and discard. Place spaghetti squash halves, cut side down on a baking sheet. Roast in the oven for about 45-50 minutes or until tender.

After the spaghetti squash is cooked and cooled slightly to allow you to handle safely, use a fork to gently scrape the insides of each shell to loosen and separate the spaghetti squash strands. Add the spaghetti squash to a large bowl and set aside. Place empty shells back onto the baking sheet and save for later.

Cook the ground beef, diced onion, and minced garlic in a skillet over medium-high heat until the ground beef is thoroughly cooked and browned. Drain off and discard any remaining grease from the meat. Combine the spaghetti sauce with the cooked ground beef and set aside.

Combine both cheeses with the cooked spaghetti squash. Add salt and pepper, to taste. Divide the spaghetti squash mixture equally among the 4 empty squash shell. Divide the meat mixture among the 4 shells, topping the spaghetti squash. Top the meat with extra shredded cheese.

Place the spaghetti squash boats into the oven and bake for about 10 minutes or until the cheese is melted and starting to brown. Remove from the oven, let cool a few minutes and serve.