Easy to whip together with what’s on hand.
Melt the butter in a saucepan over medium heat, then add the onions and cook several minutes until lightly browned.
Remove from heat and add the remaining ingredients, stirring well. Replace on heat, bring to a boil & reduce to a simmer. Cook until thickened, about 25 minutes. (It will become thicker after blending and even thicker after cooling.)
Puree with an immersion blender or in a regular blender, strain through a metal strainer if smoother sauce is desired. Will keep, refrigerated for several weeks.