In a large skillet, melt butter and olive oil over medium heat.

Add onions.

Stir occasionally.

After 10 minutes, onions will be soft but still light colored.

If browning too quickly, lower heat.

Continue to cook 40 to 50 additional minutes, stirring occasionally to scrape up and distribute tasty brown bits from bottom of pan.

Onions should be golden brown and very sweet when finished.

Add vinegar or wine to deglaze pan.

Cook until liquid is almost evaporated, stirring to incorporate.

Add salt to taste.

Leftover onions will keep 3 days in the refrigerator.