Add 1 tablespoon of butter to a large pan over medium heat. 

When butter is bubbly and frothing, add all of your sliced onion. 

Add a pinch of salt and stir everything to combine. 

Reduce heat to medium-low and cook onions, stirring often for 20 minutes. 

After 20 minutes, your onions should be tan-colored, but not browned. You might start to see dark brown spots forming on the bottom of the pan though, and if you do see that, add a tablespoon of water or two to the pan with the onions and use that water to stir the dark parts at the bottom to keep things from burning. 

Continue cooking the onions over medium-low heat for another 10 or 15 minutes. Remember: if you see dark spots or dry areas on the pan, add a tablespoon of water. The water should cook off quickly but it will help to temper the pan temperature and help the onions to cook through without burning. 

After the onions are nice and brown, remove from the pan to a sealed container and if not serving immediately, store in the fridge for no more than a week or so.