Heat a large skillet over medium heat. Add the sausage and break into bite sized pieces. Brown on one side, about 3 minutes.
Flip the sausage pieces, add the onion, and cook until the onions are softened and the sausage is browned on both sides, about 3 minutes.
Add the garlic and cook until fragrant, about 1 minute, stirring frequently. Drain any excess grease and discard, if desired.
Carefully transfer the cooked sausage and onion mixture to a 6-quart slow cooker. Add the diced potatoes, stock, water, and salt, stirring to combine.
Cover and cook on high for 4-5 hours on high, or 7-8 hours on low, or until the potatoes are tender when pierced with a fork.
Uncover and remove about 1 cup of cooked potatoes and ½ cup cooking liquid to a medium bowl. Set aside.
Add the kale, cover, and cook on high for 15-30 minutes, or until the kale is tender.
While the kale is cooking, using a fork, mash the potatoes with the cooking liquid until smooth.
When the kale is tender but still bright green, add the mashed potato mixture and cream, stirring to combine. Taste and adjust seasonings if desired.