Bring a large pot of salted water to a boil over high heat. Cook the pasta until al dente, according to the package directions. Reserve 1 cup of pasta water. Drain and set aside.
Meanwhile, heat a large nonstick skillet over medium heat. Add the sausage and cook, breaking it up with a wooden spoon, until browned, about 8 minutes. Add the garlic and cook until fragrant, 1 more minute. Using a slotted spoon, transfer the sausage and garlic to a bowl.
Add the cream and parmesan to the saucepan. Cook, stirring often, until thick enough to coat the back of a wooden spoon, about 2 minutes. Add salt and pepper to taste. Add the lemon juice, ½ cup reserved pasta water and kale and cook, stirring occasionally, until wilted, about 2 minutes.
Return the sausage mixture to the saucepan along with the drained pasta, stir to combine. Add pasta water 2 tablespoons at a time until the sauce is your desired consistency.
To serve, sprinkle the pasta with additional parmesan and red pepper flakes, if using and salt and pepper if desired.
