1.Combine the sausage, onion, tomatoes and their juices, tomato paste, garlic, 1 cup water, and ¾ teaspoon each salt and pepper in a 4- to 6-quart slow cooker. Nestle the potatoes in the liquid and top with the kale.
2.Cover and cook until the potatoes are tender, on low for 7 to 8 hours or on high for 4 to 5 hours (this will shorten total recipe time). Transfer the potatoes to a medium bowl. Add the milk, oil, and ½ teaspoon each salt and pepper to the bowl and mash.
3.Serve the stew topped with the mashed potatoes. Drizzle with more olive oil, if desired.