Saute onion in oil until softened. When nearly soft, add in dates, if using. Add greens and stir with a tong until they’re beginning to wilt. Add in the rest of the ingredients, including sugar if not using dates, cover and steam until tender, but still bright in color, about 8 to 10 minutes.
These greens tend to cook a bit more after the heat is turned off – remove them from the hot pan and give them a toss or two to bring them to a more normal serving temperature.
Serve with additional vinegar for drizzling over the top.