Meanwhile, in a saucepan, melt the butter. Add the shallot and garlic and cook over moderate heat until lightly browned, 5 minutes. Add the wine and boil for 2 minutes. Add the stock, vinegar and thyme and bring to a boil. Simmer over moderate heat until reduced to 2 1/2 cups, about 20 minutes. Strain the sauce and return it to the saucepan. Whisk in the porcini powder and simmer for 1 minute. Cover, remove from the heat and let stand for 5 minutes. Season with salt and pepper. Carve the roast and serve with the porcini jus.