In a dutch oven or large heavy-bottom saucepan, bring the stock, water, salt and pepper to a boil over high heat.
Slowly whisk in the polenta and reduce the heat to low. Simmer uncovered, stirring frequently, for 30 minutes.
Stir in the butter, taste, and add additional salt and pepper if desired. Cover and set aside. (Note: the polenta can be made 24 hours in advance. Allow to cool, then store in an airtight container in the refrigerator. Reheat during the final step of the recipe).
Meanwhile, bring a large pot of water to a boil. Have a large bowl of ice water nearby. When the water reaches a boil, add the kale leaves and cook for 60 seconds, then use tongs to transfer to the ice bath to halt the cooking process. Drain and set aside.
Heat a large skillet over medium-low heat and add 1 tablespoon of the olive oil. Add the sausage and cover. Cook, shaking the pan occasionally, for 15 minutes. (Note: if you're using sausage that's precooked, skip this step, adding the olive oil to the skillet in the next step). Once the sausage is cooked, use tongs to transfer it to a cutting board.
Cut the sausage into approximately 1/2 inch slices. Over medium heat, brown on each side for several minutes (the more caramelized the sausage, the better the flavor in the final dish will be). If at any point the bottom of the pan looks like it might burn, add 1-2 tablespoons of water, scrape up the glaze and incorporate it into the sausage. Once the sausage is browned, 3-5 minutes, remove from the pan and set aside.
Turn the heat down to low and add the remaining 1/2 tablespoon olive oil, the garlic, and the crushed red pepper. Allow the garlic to cook until fragrant, approximately 1 minute, and then add the kale to the pan. Stir to coat the kale with the seasoned oil in the pan. Add the sausage back to the pan and allow it to reheat for a few minutes if it has cooled down.
Reheat the polenta over low heat, stirring, if necessary. Serve the sausage and kale over the polenta. Leftovers can be stored in an airtight container in the refrigerator for 3-5 days.
You can reduce the total cooking time by almost half if you use two burners. On one burner, cook the polenta. On the second burner, blanch the kale and then cook the sausage.