1. Preheat the oven to 400°F (200°C). Cut the spaghetti squash in half lengthwise and scoop out the seeds.

2. Brush the inside of the squash halves with olive oil and place them cut side down on a baking sheet. Roast for 35-40 minutes, or until the flesh is tender and easily shredded with a fork.

3. While the squash is roasting, heat a skillet over medium heat. Add the chopped onion and cook until softened, about 3-4 minutes. Add the minced garlic and cook for 1 minute more.

4. In a large bowl, combine the shredded squash strands, cooked onion and garlic, cheddar cheese, half of the Parmesan cheese, heavy cream, salt, pepper, and thyme. Mix well.

5. Transfer the mixture to a greased baking dish. Sprinkle the remaining Parmesan cheese on top. Bake at 400°F (200°C) for 10-12 minutes, or until the top is golden and bubbly.

6. Remove from the oven and let cool slightly. Garnish with chopped fresh parsley before serving, if desired.