Heat a large pot over medium-low heat and add the olive oil. Add the onion with the salt, pepper and red pepper flakes and stir. Cook until onions soften, about 5 minutes, stirring occasionally. Add in the garlic and cook for 1 more minute, then add the chicken stock, increase the heat to medium-high and bring the liquid to a boil.
While the stock is heating up, heat a large non-stick skillet over medium-high heat. Remove the sausage from the casing (if it’s in casing) and add it to the skillet with a pinch of salt and pepper. Cook until the sausage is browned, about 6 to 8 minutes, breaking it into smaller pieces (or whatever size you desire) with a wooden spoon. Turn off the heat.
When the stock is boiling, add the pasta and cook for 8 to 10 minutes, just until it’s al dente. Add the kale and stir well, cooking for 2 more minutes. Stir in the cheese, then taste and season with more salt and pepper if desired. You can add all of the sausage into the soup or serve the soup into bowls and add the sausage on top per serving. Serve with extra cheese for grating. Make sure to turn the heat off underneath the soup to prevent the noodles from getting super soggy.