How to Make It
Preheat the oven to 325°. In a pepper grinder or a spice mill, coarsely grind all of the peppercorns. Transfer to a small bowl. Season the roast with salt and the pepper mixture.
In a large ovenproof skillet, heat the oil until shimmering. Add the roast, fat side down, and cook over moderate heat until browned, about 5 minutes. Turn the roast and brown it on each of the other sides, about 13 minutes longer. Set the roast, fat side up, in the skillet and add the butter. When the butter is melted, baste the roast all over. Add the thyme, bay leaves and marjoram and roast for 1 hour.
Add the garlic and shallots to the skillet and continue roasting the meat until an instant-read thermometer inserted in the center registers 130° for medium-rare, about 45 minutes longer.
In a heatproof bowl, cover the porcini with the boiling water and let soak until softened, about 20 minutes. In a large skillet, heat the olive oil. Add the white mushrooms, shallots, garlic and thyme and cook over moderately high heat, stirring occasionally, until the mushrooms and shallots are browned, about 8 minutes. Add the Armagnac and wine and boil over moderately high heat until reduced by half, 5 minutes.
Lift the porcini from their soaking liquid; reserve the liquid. Rinse the porcini and cut any large pieces in half. Carefully pour the porcini soaking liquid into the skillet, stopping before you reach the grit at the bottom. Boil the sauce until reduced by three-fourths, about 8 minutes. Add the beef broth and boil until reduced to 1 1/2 cups, about 8 minutes. Strain the sauce through a fine sieve into a small saucepan, pressing on the solids; add the porcini.
When the roast is done, transfer it to a carving board and let rest for 20 minutes. With a slotted spoon, transfer the shallots and garlic to a plate. Bring the sauce to a simmer, remove from the heat and whisk in the butter, 1 tablespoon at a time; season with salt and pepper. Carve the roast into slices. Scatter the shallots and garlic on top and serve with the porcini sauce.