Remove stems and center ribs. *See Chef’s Tip below. Cut leaves in 1/2-inch wide strips, then slice across to form 1-inch long strips. Wash well (even if they were washed before) and spin or lay out on a towel to drain a bit. It can be difficult to remove the grit in some greens.
Cut onion in half and remove papery outer layer. Thinly slice each half to form half-rounds. Heat butter in a large saucepan over medium heat. Add onion and turn down to medium-low. Cook and stir until soft and transparent, about 3-5 minutes.
While onion cooks, dice ham into small pieces, if using. Add diced ham and cook another minute. If using a ham hock, add it whole in the next step, with the water. Leave ham out altogether for a vegetarian version.
Add the water, salt (if not using ham), and vinegar. Turn heat to medium-high and bring to a boil. Add the greens and turn heat down to keep at a gentle simmer, uncovered, for about 10-15 minutes to cook off some of the water.
When greens are done, lift them out with a slotted spoon and place in a bowl. Cover to keep warm.
Continue simmering the liquids, uncovered, to reduce by half, about 5 minutes. (This intensifies the flavor.) It’s fine if there are onion pieces still floating in it. Combine the liquids and greens again.
Taste and adjust seasonings if needed. Serve with hot sauce (preferably Tabasco or a southern pepper sauce) on the side.