Cut the spaghetti squash in half and take out the seeds. Place on a shallow baking dish with about an inch of water in it. Microwave for 5 minutes. Test to see if cooked by piercing it with a fork.
Continue to microwave in 5 minute increments as needed until cooked. (You can also do the halves one at a time if they don't fit in your microwave together.) Set aside and let cool slightly.
In the meantime, heat the olive oil in a pan. Sauté garlic, onion and bell pepper and cook until softened. Add in ground beef and break up with a spatula.
When beef is cooked, add salt, pepper, chili powder and cumin, then pour in diced tomatoes. Cook until everything is heated through.
Preheat oven to 350ºF. Scrape the insides of spaghetti squash into the beef mixture and stir until all the squash is incorporated. Pour mixture back inside the two squash halves and place in a baking dish. Bake for 15 minutes, or until heated through. Top with cheese and place back in oven until cheese is melted, another 5 minutes. Serve and enjoy!