Heat a large skillet over medium heat. Add the bacon strips and cook until golden brown, 6-8 minutes. Remove to a paper towel lined plate. Reserve 2 pieces for garnish.  Crumble the remaining 4 pieces. Place the crumbled bacon, potatoes, celery, leek, garlic, stock, salt, thyme sprigs, and pepper in the base of a 6- to 8-quart slow cooker. Put the lid on the slow cooker, and cook on high for 4 hours (or low for 6 to 8 hours) until the potatoes are tender when pierced with a fork.  Remove the thyme sprigs and discard. Use an immersion blender to blend the soup to a semi-smooth consistency, leaving small chunks of potato and vegetables for texture. Stir in the sour cream. (Add 1/4 - 1/2 cup of water, to thin the soup out to your desired consistency). Serve in bowls, garnished with cheese, chives, additional sour cream, and the reserved bacon slices crumbled over top.