1. Working in batches, stack greens; cut crosswise into 2-inch wide strips. Then cut crosswise into 2-inch pieces. Transfer to a large bowl of cold water to remove grit. Drain. Repeat until greens are clean. Set aside.
2. Heat oil in a large skillet on medium-high heat. Add onion and bacon; cook 3 to 4 minutes or until onion is translucent. Add greens; cook, stirring frequently, until greens begin to wilt and are reduced in volume.
3. Increase heat to high. Add vinegar. Cook 1 minute, scraping up brown bits from bottom of skillet, until vinegar has evaporated.
4. Add stock; reduce heat to low. Simmer, covered, 12 to 14 minutes or until greens are just tender. Divide evenly between 6 plates and serve.
Nutritional information per serving: Calories 113; Total Fat 8g; Saturated Fat 2g;