This sauce goes great on Meatloaf.
3 tablespoons butter
2 cloves garlic, minced
2 portobello mushrooms, cut into 3/8-inch cubes
1 16-ounce can low-sodium beef broth
1 ounce dried porcini mushrooms, ground to dust in a coffee grinder
1 tablespoon Wondra flour
Heat 2 tablespoons butter in a sauté pan, add the garlic and portobello, and sauté until soft.
Bring the broth to a boil in a saucepan, and reduce by a quarter. Remove from the heat and add the ground porcini.
Mix the flour and 4 tablespoons water in a bowl, stirring until combined. Add the sautéed mushrooms and flour to the porcini broth, set the saucepan over low heat, and stir until the broth thickens. Stir in the remaining butter just before serving.
Source: NY Mag