Bring a large pot of salted water to boil. Cook the pasta according to package instructions, until just al dente. Reserve a ½ cup of pasta water. Drain and set aside.
Meanwhile, heat a large skillet or dutch oven over medium high heat. Add the sausage, using a spoon to break it up as it cooks. Cook for about 5 minutes and add in the chopped onion. Continue cooking until the onion is softened and the sausage is no longer pink.
To the skillet, stir in the garlic, chopped kale, lemon juice, salt, and pasta water. Continue cooking until the kale softens and shrinks in size; about two minutes. Add in the cooked pasta and parmesan and stir until the mixture is fully combined.
Be sure you are using a large skillet or dutch oven to make this dish. You will need a lot of room to toss the sausage and kale with the pasta, so the bigger the skillet the better.
Use a good quality Italian sausage and be sure to remove the casing. Crumble the sausage into the pan and use a spoon or spatula to break it up as it cooks. Sweet or spicy Italian sausage works.
Save a cup of pasta water before draining the cooked pasta. Use the water to create a sauce and deglaze the pan.