Heat a large saucepan over medium heat and melt 1/4 cup of butter to begin making demi glace. Stir occasionally until the butter is completely melted. Reduce the heat slightly if the butter begins to brown.
Slowly sprinkle flour over the melted butter. Use a wire whisk to incorporate the flour into the butter and prevent it from clumping. The butter and flour will form a roux and look like a thickened paste when completely combined.
Pour broth into the pan. Use a whisk to stir the roux of flour and butter off the bottom of the pan and mix it into the broth. Whisk the mixture until the roux is fully mixed into the broth and all clumps have been broken down.
Stir red wine into the broth using a circular motion. Be sure the red wine is thoroughly incorporated into the mixture.
Allow the red wine demi glace to simmer at low heat for 1 hour. Stir the mixture occasionally. Reduce heat slightly if there is any buildup on the bottom of the pan. This is a sign that your sauce is too hot and could burn.
Check the broth and wine after 1 hour to see if it has reduced in volume. The liquid should have reduced to approximately half its original volume in the pan. This concentrates the flavor and thickens the sauce.
Dip a spoon into the demi glace, then turn it over and look at the back. The demi glace is ready if it glazes the back of the spoon and stays on it. Simmer the sauce for an additional 20 minutes if the demi glace is still runny when you test its consistency. Then perform the same consistency test to check the thickness of the sauce.
Melt an additional 1/4 cup of butter slowly into the demi glace over low heat. Stir the sauce continuously as the butter melts so it mixes completely into the sauce.
Use the finished demi glace as a glaze or drizzle over steaks or roasts. You can also serve demi glace on the side like a gravy. Demi glace is a concentrated sauce with strong flavor, so use it in moderation until you know your tastes.
