Heat a cast iron skillet to medium heat and add the sliced onions with equal parts butter and oil.
1-2 Onions, Butter, Vegetable Oil
Turn the heat down to low when the onions soften and sprinkle with kosher salt and let the onions cook, stirring occasionally, until golden brown (about 15-20 minutes).
Kosher Salt
Make the Sauce
Whisk all the ingredients for the sauce together in a small bowl and set aside.