Butternut Squash Agnolotti with Kale in a Sage Brown Butter Sauce

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.