Over medium heat, melt 1 tablespoon butter in a skillet.
Add the onions and let them cook for 20 minutes while stirring occasionally.
Add the sugar and cook for 2 minutes.
Stir in the vinegar then cover the skillet, turn down the heat, and cook for 25 minutes. You can add up to 1/4 cup of water if the pan starts to dry, do it 1 tablespoon at a time.
Stir in the rest of the butter.
Add the salt and pepper.
Sprinkle with parsley and serve.