3 tablespoons butter
1/4 cup finely chopped shallots
1 pound assorted mushrooms, cut into pieces
1 cup port wine
1/4 cup coarse-ground Dijon mustard
1 (14 ounce) can beef broth
1 tablespoon cornstarch
2 tablespoons water
1 tablespoon cold butter (optional)
Melt 2 tablespoons butter in a large skillet over medium-high heat. Stir in the shallots, and cook for 2 minutes to soften slightly. Add the mushrooms, and continue cooking until the mushrooms have released their liquid and are tender. Remove the mushrooms from the skillet and set aside.
Pour the port wine into the skillet and bring to a boil over high heat. Boil until the port has reduced and is almost syrupy, 5 to 7 minutes. Whisk in the mustard and beef broth. Dissolve the cornstarch into the water, and whisk into the boiling sauce. Stir until the sauce has thickened, about 30 seconds. Remove the skillet from the heat, and whisk in the remaining 1 tablespoon butter until it has melted into the sauce. Stir the cooked mushrooms back into the sauce and serve.