Remove the zest from half of the orange, cut into ¼ inch strips and reserve. Squeeze the juice from the orange and save that too.
Heat up a sauce pan over medium heat. When hot, add the butter and then the shallots. Saute the shallots until translucent, about 3 minutes.
Add the orange juice, vinegar and orange liqueur (optional). Let this reduce for 2 minutes.
Add the demi glace and reserved orange zest to the sauce pan. Let this simmer and reduce down until the sauce is thick enough to coat the back of a spoons.
Taste and adjust seasonings with salt and pepper.
Strain the sauce through a fine mesh strainer or cheesecloth.
Serve with duck, chicken or trout.