Caramelized Onion and Spinach Grilled Cheese

I was making grilled cheese for dinner one night because I needed an easy dinner night.  After making a couple standard cheddar cheese sandwiches, I had an idea.  I had sweet onions and spinach and decided to take it up a notch.  The resulting caramelized onion and spinach grilled cheese was delicious.  So good I have found myself wanting more...in fact, I just had this for lunch.  

Ingredients:
butter
sweet onions, sliced 
fresh spinach
swiss cheese or your favorite cheese
sourdough or another nice variety of bread

1.  Saute some sliced onions with a little butter over medium heat.  (A slice or two of onion for each sandwich.)  Cook until the onion starts to turn golden brown and caramelize.  Break the onion slices apart as they cook.  Add a handful of fresh spinach.  Continue to cook just until the spinach wilts.  

2.  Remove the onion and spinach from the skillet.  Place a pat of butter in the center of the skillet.  Add a slice of bread then place the onion spinach mixture on the bread.  Add the cheese ( I use 2 slices) then top with the second piece of bread.  Add a little butter on top of the sandwich so that when you flip it over, the other side will have a buttery crust too.  

3.  Cook on each side until golden brown.