Heat oil over medium heat in a 12″ non-stick skillet until shimmering. Add the onions, salt and pepper and cook, stirring occasionally, until translucent and just starting to brown, about 5-7 minutes. Add the remaining ingredients and bring to a boil. Reduce to a simmer and cook until almost all of the liquid has evaporated and the mixture is jammy, about 15 minutes. Taste and adjust seasoning as needed. Transfer to an air-tight container and cool to room temperature. Store in the refrigerator for up to 2 weeks.