Combine brown stock and Spanish sauce in a large saucepan. Bring to a boil over medium heat, then reduce heat and simmer gently for about 1 to 2 hours, until sauce is reduced by half and reaches a smooth consistency. If desired, add red wine halfway through cooking for a richer taste.
Season with salt and pepper to taste. Strain the sauce through a fine sieve before serving to obtain a smooth texture.
The demi-glace is ideal with roast meats, steaks or simmered dishes!
