Pour in the stock and beer and add the bay leaf and seasonings. Bring to a boil, and turn the heat to medium low, and cover. Simmer until reduced by half.
2 cup vegetable stock, 1½ cups dark beer, 3 cloves, 1 bay leaf, 2 teaspoons brown mustard
If the sauce is too thin, bring the pot back to a boil, make a slurry of 2 teaspoons of arrowroot powder and a little water. Gradually add the slurry while stirring until thickness is just where you want it.
2 teaspoons arrowroot
Notes
If you don't have arrowroot, substitute potato starch or corn starch.