In a large, heavy-bottomed saucepan for which you have a lid (preferably stainless steel or unfinished cast iron), melt the butter over medium-low heat. Once the butter is melted and getting foamy, add all of the sliced onions along with a generous pinch of salt.

Cover the pot with a lid and allow the onions to release their moisture. Keep the pot covered, checking occasionally and stirring to prevent browning, until the onions have reduced in size by about half - about 20-30 minutes.

Remove the lid and cook, stirring occasionally to ensure there is no browning or burning, until the onions soften completely and begin to take on a uniform brown colour - about 20 more minutes. This will happen very gradually. If you notice some onions browning around the edges or any fond is collecting on the bottom of the pan, add a few splashes of cold water to prevent burning.

Cook the onions, stirring frequently and adding more water as needed, until they are a uniform dark amber colour, about another 10-15 minutes. Remove from the heat and serve.