Melt the butter in a saucepan over medium heat. Add the onions and cook until soft, about 3 minutes. Add the yellow mustard, lemon juice, brown sugar, honey, black pepper, mustard powder, salt and hot sauce and simmer on low until slightly thickened, about 15 minutes.
Puree in a blender if you prefer a smoother sauce, or leave as is if you don't mind a little texture. Use immediately, or refrigerate for up to 2 days.