Cut the kale from the stem and chop into smaller pieces
Peel and cut the onion into small dices.
In a large pot or dutch oven, melt the fat over medium heat.
Add the onion and saute until translucent.
Gradually add the kale and saute while stirring. The kale will shrink a lot!
Add the broth and mustard. Cover the pot with a lid and turn the heat to low. Simmer for about 1.5 hours.
Check occasionally to make sure that the kale doesn't get too dry. If needed, add more broth.
At the end of the cooking time, add the rolled oats and cook for another 10-15 minutes. Season to taste with salt and black pepper.
Serve with red smoked sausage (Polish Kielbasa) or your favorite meat and boiled potatoes.