Remove and discard stems from greens. Wash leaves thoroughly, and cut into 1-inch-wide strips; set aside.
Pulse onion in a food processor 3 or 4 times or until minced.
Bring onion and next 4 ingredients to a boil in a Dutch oven. Add greens and bell pepper; cook, covered, over medium heat 45 minutes to 1 hour or until greens are tender.