Heat a skillet over medium-low heat and add the oil or butter.
Add the diced onions, a large pinch of salt, and a small pinch of sugar.
Cook slowly, stirring occasionally, for 25–30 minutes (or until the onions are a deep golden-brown and jammy).
Add 1-2 tbsp of them to a bowl with the rest of the sauce ingredients and whisk until smooth and fully combined.
Season to taste & refrigerate for at least 30 minutes so the flavors meld together.
