Heat a pot or large pan over medium-high heat. If using lean sausage, heat about ½ a Tablespoon of oil in the pan. Slice the sausage into thin rounds and brown on both sides. (Or, remove the casing, crumble into the pan, and brown.)While the sausage browns, wash and chop the kale.Slice the onion and add to the pan. Sauté for about 2 minutes, then add the garlic, oregano, and red pepper flakes, and sauté about 30 seconds more.Drain and rinse the beans. Add the beans and tomatoes to the pot and stir well. Place the clean & chopped kale on top and cover. Cook, covered, for about 10 minutes. Stir to combine the kale and cook another 2 minutes.Add additional salt, pepper, and red pepper flakes as desired and serve.