In a large saucepan, heat the oil, add the garlic and onion, and cook until the vegetables are soft, about 2 minutes.
Add the demi-glace and the next seven ingredients, whisking to blend them together well, then bring the sauce to a simmer over low heat. Cook for 20-30 minutes, stirring occasionally. Season the sauce to taste with salt and pepper.
Additional Tips
The sauce is even better if you make it 2-3 days before you use it, so the flavors blend and develop. Store the sauce covered in the refrigerator.
