Prime Rib and Potatoes
For the roast, pulse the porcini mushrooms in a spice grinder a few times until they become a mixture of small pieces and dust. Transfer to a bowl and combine with 1 3/4 teaspoons of the salt and 3/4 teaspoon of the pepper.
Rub the mixture over the entire rib roast and transfer it to an oiled rimmed baking sheet.
Combine the potatoes, onion wedges, oil and the remaining 3/4 teaspoon salt and 1/4 teaspoon pepper in a bowl; toss gently to coat. Arrange vegetables on the baking sheet around the roast and let stand at room temperature for 20 minutes.
Preheat the oven to 350°F.
Cook the rib roast until and instant read thermometer inserted into the thickest portion of the meat registers 115°F, about 1 hour 45 to 1 hour 50 minutes.
Remove from the oven and loosely tent with aluminum foil; let rest 25-30 minutes before slicing.
While the roast cooks make the Mushroom-Shallot Reduction.
Mushroom-Shallot Reduction
Soak the dried porcini mushrooms in 1 1/2 cups of hot water until softened, about 20 minutes. Transfer the porcini mushrooms to a cutting board with a slotted spoon and coarsely chop them. Reserve 1/2 cup of the porcini soaking liquid.
Melt 2 tablespoons of the butter with the oil in a large skillet over medium-high heat.
Add the white mushrooms, shiitake mushrooms, shallots, thyme and salt; cook, stirring occasionally, until the mushrooms just start to brown, about 9-10 minutes.
Stir in the porcini mushrooms and cook, stirring, 1 minute.
Add the madeira and reserved porcini liquid, bring to a boil and cook until the liquid is absorbed, about 1 1/2- 2 minutes.
Pour in the chicken stock, bring to a boil and cook until the liquid is slightly thickened, about 6 minutes. Remove from the heat and add the remaining 2 tablespoons butter, stirring constantly, until melted. Season with the pepper and serve over the sliced rib roast.
