Slice onions thinly and evenly
Heat a pan over medium-low heat
Add butter, oil, or both
Add the onions and a pinch of salt
Stir to coat the onions in fat
Cook slowly, stirring every few minutes
Add a splash of water if the pan gets too dry
Lower the heat if the onions start to brown too fast
Continue cooking until the onions turn deep golden brown
Cook until soft, sweet, and jammy
Use immediately or store for later