Finely chopping the kale helps it cook quickly while retaining its pleasant chewiness. Slideshow: More Spaghetti Recipes
Heat the olive oil in a large skillet over moderately high heat. Add the sausage and cook, stirring occasionally, until well browned, about 10 minutes. Add the kale and the garlic and cook, stirring occasionally, until it’s bright green and wilted, about 5 minutes. Season to taste with salt and pepper.
Meanwhile, cook the spaghetti in a large pot of boiling well-salted water until al dente. Set aside 1 cup of the pasta cooking water, and then drain the pasta well. Add the spaghetti to the skillet and cook, adding the pasta cooking water as needed to loosen the sauce, and tossing to combine well. Taste and adjust the seasonings.
Serve immediately, with finely grated Parmesan cheese on the side.