Preheat your oven to 400°F and lightly coat a 1.5-quart baking dish with olive oil cooking spray.
Microwave the spaghetti squash:
Using a sharp knife, carefully pierce the squash in several places. Microwave for 6 minutes, then flip and cook for an additional 6 minutes until it becomes tender. Let the squash sit for about 5 minutes to ensure it finishes cooking.
Once it's cool enough to handle, cut the squash in half lengthwise with a sharp knife. Scoop out the seeds and pulp from the center. Use a fork to gently pull apart the flesh into long strands, placing them in a bowl. Set aside.
Prepare the casserole:
In a large mixing bowl, whisk the egg lightly. Add the sour cream, salt, pepper, thyme, and minced garlic. Stir until the mixture is smooth and well-blended.
Fold in the cooked spaghetti squash and shredded cheddar cheese until evenly mixed.
Pour the combined ingredients into the prepared baking dish. Place in the oven and bake for about 40 minutes, or until the top turns golden brown and fills the kitchen with a delightful aroma.
Once done, allow the casserole to rest for about 10 minutes before serving. Scoop portions onto plates or slice into squares to serve.