Rosemary sherry reduction

All great dishes include some form of a great sauce, and here is a basic recipe for a nice demi-glace based reduction. I usually make demi glace in a largish batch and then freeze it in 1/2 liter (2 cup) bags for future use.

2 shallots, sliced
1 tsp chopped garlic
1 tbsp cup butter
1/4 cup red wine vinegar
1/2 cup sherry
1 branch rosemary
2 branches thyme
2 cups demi glace, (beef or veal)

Sauté shallots and garlic in butter until tender
Add red wine vinegar and reduce until syrupy (au sec)
Add sherry and herbs and reduce by half
Add demi glace and simmer for 1 hour, strain, and adjust seasoning