Wash the spaghetti squash. Pierce the rind 4-5 times with a sharp knife. Place in oven-proof pan. Bake in preheated 375 degree oven until soft, 1 - 1 1/2 hours. Remove from oven and reduce heat to 350 degrees. (Alternatively, place pierced squash on paper-towel lined plate and microwave 10-12 minutes.).
When the squash is cool enough to handle, slice lengthwise. Scoop out seeds and discard. Fork the squash into strands and place in buttered casserole dish.
While squash is baking, mix together soup, sour cream, chopped onion, garlic, and 1/4 cup of cheese.
When strands of squash have been placed into casserole dish, toss with melted butter and sprinkle with salt and pepper to taste (I use almost no salt as the cheese tends to be salty enough).
Pour soup mixture over squash and stir well. Top with remaining cheese.
Bake at 350 degrees for 30 minutes or until cheese is browned.