"First, make the reduction. Melt 1 tablespoon of the butter in a large saucepan over medium-low heat. Add the diced shallot and cook untile softened, about five minutes. Then, add all of the port (yes, a whole bottle!), and the thyme sprigs, and bring to a boil over high heat. Watch for the boil, then reduce the heat to maintain a brisk simmer. Cook until the port has reduced to the consistency of corn syrup, about 30 minutes. You should have about 1/2 cup or so including the diced shallot.At the same time, bring the chicken broth to a simmer in a small saucepan. Once it's brought a simmer, turn off the heat, and add the package of porcini mushrooms. Here, you're creating a mushroom-enriched stock that will add a nice earthy base to the sauce. Let the mushrooms soak for 15-20 minutes. with a straining or slotted spoon, lift the porcini to a small bowl. Strain the sooaking liquid through a coffee filter, paper towel or cheesecloth to remove any dirt sediment that came from the porcini mushrooms. Save the stock -- that's what you're going to use in the next step.Add the enriched chicken stock and half of the porcini mushrooms (diced) to the port reduction. (You can use the other half of the porcini mushrooms in a beef or mushroom-cap stuffing.) Bring the sauce to a boil over high heat, then reduce to maintain a brisk simmer. Cook until it's reduced to about 1 1/3 cups, which is about 15 minutes. Strain again through a fine strainer, pressing on the solid mushrooms and shallots. Add salt and pepper to taste. You should have a little over 1 cup of the sauce. The sauce can be refrigerated at this point and held for up to 2 days, if you seal it carefully with plastic wrap.Twenty minutes before plating your dinner, bring the sauce back to a low simmer, stir. In a separate bowl, mix the flour and butter into a paste, then WHISK rapidly into the simmering sauce to thicken it. You'll need to whisk for 2 minutes or so to give the sauce a single consistency. Raise the temperature to a simmer, and maintain that while whisking for 2-3 minutes to remove any flour flavor. Add a teaspoon or so of fine balsamic vinegar, and taste a few times. You're done! Serving options: Can be plated first, with filet mignon beef medalions on top, or poured over steak. "