Heat your largest skillet, either stainless steel or cast iron, to medium heat.
Melt the butter and add the olive oil.
Cook the onions in the pan, stirring every five minutes or so.
The onions will begin to brown at around the 20 minute mark. However, the longer you leave them cooking, the darker and sweeter they will turn.
Once they turn the color you desire, pour the chicken stock in the pan.
Deglaze the pan by scraping the burned bits, or fond, with a wooden spatula.
Add salt to taste.
Serve immediately or wait a few minutes - they will turn jammy!