In a large frying pan, melt butter over medium heat. Add onions and cook, stirring often, until they start to turn golden brown (be careful not to burn them), about 10 minutes.
Add white wine and vinegar and cook, stirring occasionally, until liquid is reduced to a glaze, about 3 minutes. Add chicken broth and salt to taste. Reduce heat to medium-low and simmer until liquid is nearly evaporated and onions are tender and a rich brown, 10 to 20 minutes.