Directions

1. Working in batches, stack greens; cut crosswise into 2-inch-thick strips. Gather strips; cut crosswise into 2-inch pieces. Transfer to a large bowl of cold water; swish to remove grit. Transfer greens to a colander using a slotted spoon; let drain. Repeat until greens are free of grit.
2. Heat oil in a very large skillet over medium-high heat. Add onion and bacon; cook until onions are translucent, about 4 minutes. Add greens; cook, stirring, until greens begin to wilt and are reduced in volume.
3. Raise heat to high; add vinegar. Cook, scraping up brown bits from bottom of skillet, until vinegar has evaporated, about 1 minute.
4. Add stock; reduce heat. Simmer, covered, until greens are just tender, 12 to 14 minutes. If making ahead, refrigerate, covered; reheat over low heat.