Bring a large pot of salted water to a boil. Wash and dry all produce. Halve, peel, and thinly slice half the onion (all for 4 servings). Remove and discard any large stems from kale. Peel and mince or grate garlic. Pick sage leaves from stems; roughly chop leaves.

Heat a large drizzle of olive oil in a large pan over medium-high heat. Add sliced onion and cook until slightly softened, 3-4 minutes. Add half the kale (all for 4 servings); season with salt and pepper. Cook, stirring, until tender, 5-7 minutes. TIP: If necessary, add a splash of water to help soften kale. Stir in garlic and another drizzle of olive oil. Cook until fragrant, 30 seconds. Remove from pan; set aside.

Once water is boiling, add agnolotti to pot. Lower heat slightly and let water come to a gentle boil. Cook until agnolotti are tender and floating to the top, 3-5 minutes. Reserve 1 cup pasta cooking water, then drain.

While pasta cooks, melt 1 TBSP plain butter (2 TBSP for 4 servings) in pan used for kale over medium heat. Add sage and walnuts. Cook, stirring, until walnuts smell toasty and butter is foamy and flecked with amber brown bits, 1-2 minutes.

Stir drained agnolotti, kale mixture, and ¼ cup reserved pasta cooking water (⅓ cup for 4 servings) into pan. Increase heat to high and let bubble until liquid is reduced by half, 2-3 minutes. TIP: If needed, add more reserved pasta cooking water a splash at a time until agnolotti are coated in sauce. Stir in garlic herb butter and half the Parmesan (save the rest for serving). Taste and season with salt and pepper.

Divide pasta between bowls. Sprinkle with remaining Parmesan and serve.