Melt the butter in a heavy skillet and sauté the onion until transparent. Add the carrots and cook until soft.
Place garlic, peppercorns, and bay leaf in cheesecloth bag. Add to pan along with 1 cup of beef broth. Boil until broth has been slightly reduced, about 10 minutes, then season with thyme and salt.
Remove cheesecloth bag and put mixture through a coarse strainer. Return to pan.
Meanwhile, dissolve flour in the other 1/4 cup of broth and add gradually to sauce. Stir constantly until sauce thickens, then add the wine. Reduce heat and allow to simmer until ready to serve, and then sprinkle chopped parsley on top.
Use a quality beef broth if not making your own such as More Than Gourmet Glace de Veau (veal stock).