Start by caramelizing the onions, as this step can take up to half an hour. Heat 1 Tbs. olive oil in a large skillet over medium heat. Add onions, stir to coat with oil, and season with salt and pepper. Spread onions out evenly across the pan and let cook, stirring occasionally, until onions are soft and caramelized, about 25-30 minutes. Don’t stir the onions too often so they can brown, but don’t leave them alone long enough to burn or stick to the pan. You may need to reduce the heat or add a splash of water to deglaze the pan a few times to prevent onions from burning. Once they’re done, set aside in a small bowl. This step can be done ahead of time and onions can be warmed up in the microwave before topping burger.