Place all ingredients in a medium mixing bowl and smash together with the back of a spoon. Taste test it by adding a small amount to a piece of plain bread or cracker. Adjust seasoning as desired and do another taste test. To store, form into a log using plastic wrap and store in the freezer until ready to use.
If you need to quickly bring cold butter to room temperature, cut it into 1" pieces and place evenly on a dinner plate. Cook in the microwave in 30 second increments at 20% power. The butter should be soft to the touch but not runny.
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.