These greens are cooked long and slow.
2 T vegetable oil
1/2 c finely chopped onion
1/2 c finely chopped celery, optional
12 c chopped mixed greens
1 T apple cider vinegar
1/2 t red pepper flakes
1/2 t salt
2 c water
1. Heat the oil in a Dutch oven over medium heat. Add the onion and celery, cook 5 minutes, until tender, stirring occasionally.
2. Stir in the greens. Cover and cook 10 minutes, until wilted, stirring occasionally.
3. Add the vinegar, red pepper flakes, and salt. Cover and cook 5 minutes.
4. Add 2 cups water, reduce the heat, and simmer 1 and 1/2 hours, adding more water if necessary.