Wash and dry all produce. Bring a large pot of salted water to a boil. Halve, peel, and thinly slice shallot. Remove any stems or large ribs from kale. Mince or grate garlic. Pick sage leaves from stems; discard stems. Roughly chop leaves.

Heat a large drizzle of olive oil in a large pan over medium-high heat. Add shallot and cook until beginning to soften, 1-2 minutes. Add half the kale (all the kale for 4 servings). Season with salt and pepper. Cook, stirring, until tender, 5-7 minutes. Add garlic and another drizzle of olive oil. Cook until fragrant, 30 seconds to 1 minute. 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. Carefully scoop out and reserve 1 cup pasta cooking water, then drain.

Meanwhile, melt 2 TBSP plain butter (4 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 on top and flecked with amber brown bits, 1-2 minutes.

Stir 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: Add a splash or two of remaining pasta water if sauce seems dry.) Stir in garlic herb butter and half the Parmesan. Taste and season with salt and pepper.

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