Place the turkey bacon, potatoes, diced onion, and chicken stock into a 4-quart or larger slow cooker. Stir well, cover, and cook on LOW for 6-8 hours or HIGH for 3-4 hours until the potatoes are tender.
In a small bowl, whisk together the evaporated milk and cornstarch until smooth and lump-free. During the last 30 minutes of cooking, add this mixture to the slow cooker along with the softened cream cheese, shredded cheddar, salt, black pepper, and celery seed. Stir thoroughly, re-cover, and continue cooking for 30 minutes.
Uncover the slow cooker and use a potato masher to mash about half of the potatoes, creating a thicker texture while retaining some chunkiness.
Serve immediately, garnished with your choice of toppings such as additional cheese, green onions or chives, and extra bacon. Leftovers can be stored in an airtight container in the refrigerator for up to 3-4 days.