Wash the spaghetti squash thoroughly under running water and pat it dry with a towel.
Pierce the spaghetti squash several times with a sharp knife or fork to allow steam to escape during cooking.
Place the whole spaghetti squash in the slow cooker.
Cover the slow cooker with its lid and cook on low heat for 6-8 hours or on high heat for 3-4 hours. The cooking time may vary depending on the size of the squash and the heat settings of your slow cooker.
After the cooking time is up, carefully remove the spaghetti squash from the slow cooker using tongs or a spatula. It will be very hot, so be cautious.
Cut the spaghetti squash in half lengthwise and scoop out the seeds using a spoon. The flesh should be tender and easily separated into spaghetti-like strands.
Transfer the spaghetti squash strands to a serving dish. Discard the outer skin.
Add butter, grated Parmesan cheese, salt, and pepper to the spaghetti squash. Toss everything together gently to coat the strands evenly.
Sprinkle chopped fresh parsley over the spaghetti squash for garnish.
Serve immediately while warm, and enjoy your delicious slow cooker spaghetti squash with parmesan cheese, butter, salt, pepper, and parsley!