Sauteed Kale

This was our first experience with kale, and a huge success.  Ben said I could make this once a week, he loved it!  Elliot ate it up too. 

1 T. oil or butter
1/2 cup red onion, thinly sliced
12 cups kale, stemmed and chopped (10 oz)
  **be sure to get rid of all the woody stems or it will ruin the dish**
Apple cider vinegar to taste
Salt to taste

Heat oil in large saute pan over medium-high.  Add onion, saute for 3 minutes.  Add greens and saute for 2-4 minutes, pressing down on them with a spoon until they start to wilt but are still bright green.  Season with vinegar and salt.  Optional - you can also add some garlic, red pepper flakes, and white cannellini beans.  Or sub mustard greens for kale.