Slice a full white onion
In a pan on medium/high heat add in oilve oil
toss in sliced onions then add salt and pepper
Allow onions to cook down then add in butter and let that melt
Pour in red wine vinegar then allow it to cook down. Most of it will cook out but keep a eye on the onions so they don't burn. If they begin to look dry which they shouldn't add in more oil or butter not vinegar.