Preheat the oven to 400 degrees. Cut open the spaghetti squash and remove seeds and sinews. Brush the insides with 1 tsp olive oil and sprinkle with salt and pepper. Place, cut side down, on a baking sheet and roast for 50-60 minutes, until golden. Remove from the oven and let cool.
When the spaghetti squash is almost done roasting, heat the remaining 1 tsp of olive oil in a large, oven-safe skillet. Add the onion and season with salt and pepper. Cook, stirring frequently, until softened and beginning to turn golden brown. Push the onions to one side of the pan and add the ground beef. Season with salt and pepper, and cook, breaking apart as you go, until browned. Drain excess fat, if needed.
Combine the onions and beef in the pan and add the garlic. Cook, stirring frequently, until the garlic is fragrant -- about 1-2 minutes.
Stir in the crushed tomatoes. Cover, and let cook for 10 minutes. Taste, and adjust seasoning as needed.
Preheat the oven to 325 degrees.
Using a fork, break apart the spaghetti squash into strands. Place in a colander and press to drain. It can be very watery, so you want to drain as much as you can.
Add the spaghetti squash to the bolognese sauce. Stir well to combine. Sprinkle with mozzarella cheese. Slide into the oven and cook for an additional 10-15 minutes, until bubbly and the cheese is melted.
Remove from the oven and let cool for about 10 minutes before serving.