Melt the Butter and mix it with the Olive Oil then toss the sliced Onions in it
Pre-heat your pan to Medium Low then add the coated Onions, Salt and Pepper
Cook these on medium-low uncovered for about an hour stirring occasionally until they reach a golden caramel color. You have to use your own judgement here, the cooking time is an estimate but will change depending on onion thickness, pan heat, etc…
Notes
You can add a few tablespoons of sweet sherry or Marsala wine toward the end of cooking to give your caramelized onions an extra dimension of flavor.