Bring a large pot of salted water to a boil. Wash and dry produce. • Halve, peel, and finely chop onion. Remove and discard any large stems from kale; chop into bite-size pieces if necessary. Peel and thinly slice garlic. Quarter lemon.
Once water is boiling, add cavatappi to pot. Cook, stirring occasionally, until al dente, 9-11 minutes. • Reserve 3⁄4 cup pasta cooking water (1 1⁄2 cups for 4 servings), then drain.
Heat a large drizzle of oil in a large pan over medium-high heat. Add sausage* and onion; season with salt and pepper. Cook, breaking up meat into pieces, until sausage is mostly cooked through and onion is softened, 3-5 minutes. • Stir in kale and garlic; cook until sausage is cooked through and kale is wilted, 3-5 minutes. TIP: Add kale in batches if necessary.
Reduce heat to medium. Stir in cream sauce base, sour cream, stock concentrate, 1⁄4 cup reserved pasta cooking water (1⁄3 cup for 4 servings), and a big squeeze of lemon juice. Cook, stirring occasionally, until thickened, 2-3 minutes. Season with salt and pepper.
Reduce heat to low. Stir in drained cavatappi, half the Parmesan (use the rest for serving), 2 TBSP butter (4 TBSP for 4 servings), and a squeeze of lemon juice. TIP: If needed, stir in more reserved cooking water a splash at a time until pasta is coated in a creamy sauce.
Divide pasta between bowls; top with remaining Parmesan. Serve with remaining lemon wedges on the side.