Trim any woody stems from the bottom of the greens. Tear the center ribs from the leaves and thinly slice the ribs. Heat a large sauté pan over medium-high heat, then add the oil and swirl to coat the pan. Add the sliced ribs, onion, and a pinch of salt; stir. Sauté until the onion is softened and beginning to brown, about 4 minutes.
Roughly chop the remaining greens and add them to the