Slice three large onions in half and cut into thick half-moon slices.
Heat the butter over medium in a large skillet, add the onions, and stir to coat the onions with the butter.
Allow to cook on low/medium-low for about 10 minutes, stirring occasionally. Add the salt (and a little water if your onions are drying out) and mix well.
Continue to cook, stirring regularly (let them get brown, but not burned) for about 30 more minutes.
Remove from the heat and allow to cool.
Store in the refrigerator in a covered container or freeze until needed.