Slice the onions thin. You can use a mandolin for this chore, or if you have the knife skills, slice them by hand. They do not need to be “exact”, just thin.
Place the butter in a large skillet, over medium low heat and heat until just sizzeling.
When the butter has melted add the onions. Mix well with tongs. Reduce the leat to between medium to low.
Add the salt and pepper, mix with tongs.
The process of caramelizing these onions will take at least 3 hours and up to 4 hours. Low heat, slow caramelizing is the key. Turn them regularlly. This is not a chore that you can just leave on the stove. It must be watched to prevent burning and for even caramelization.
The caramelized onions will be sweet and will not have any strong onion taste. They should be browned.
The onions will lose water and shrink, so you will end up with much less volume than with which you started.
Notes
*I prefer sweet or Vidalia onions, but regular yellow or white work as well.