Cook. Get a large frying pan. It is important that you spread the onions out over a large surface, so don't do this in a saucepan or a small frying pan. And non-stick is best. Over a low heat, melt the butter, stir in the onions til coated, and add the salt. Cover for 15 minutes so they can steam with their internal moisture. Uncover, turn the heat up to medium, and let them cook, stirring every 5 minutes or so, until they are brown all over but not burned. The idea is to slowly extract most of the moisture, burn off the acrid stuff that makes you cry, and turn the copious natural sugars to caramel. Use immediately or keep in the fridge for a week or two.