To make the sauce: In a medium saucepan over medium-high heat, melt the butter. Add the onion and cook until soft, about 5 minutes, stirring occasionally. Add the garlic and cook until light brown, about 1 minute, stirring occasionally. Add the remaining sauce ingredients. Whisk to combine. Bring the sauce to a boil, then lower heat to a simmer. Cook for 5 to 10 minutes. Pour about half the sauce into a small bowl, reserving the other half in the saucepan.