When my mom visits in the summer, the first thing she says is lets go get some Swiss chard.  Here's a version of how she likes to cook it:

2 TBLS cooking oil
1 head of garlic, peeled and chopped
½-1 onion, chopped
1 bunch of Swiss chard, torn into pieces
salt to taste
dash of vinegar

Heat oil on medium.  Sauté onion until soft, about 10 min.  Add chard, garlic, & salt.  Sauté gently, covered, adding a little water if necessary, until very tender, 15-20 min.  Serve with vinegar.