Ground porcini mushrooms add tons of flavor to this roast.
In a spice mill, grind the porcini and peppercorns into a powder. Transfer to a small bowl. Mix in the salt. Rub half the porcini powder all over the roast. Place the roast, fat side up, on a wire rack set inside a large roasting pan. Let sit at room temperature for 1 hour so the rub will absorb into the meat. Rub the remaining porcini powder over the roast.
Preheat the oven to 450°. Cook the roast for 20 minutes. Reduce the oven temperature to 350°. Cook the roast until an instant-read thermometer inserted into the center registers 125° for medium-rare, about 1 hour and 15 minutes. Transfer the roast to a cutting board. Let rest for 30 minutes.
While the roast rests, in a large skillet, heat the oil over medium-high. Add the cremini and chopped thyme. Cook, stirring often, until browned, 5 to 8 minutes. Add 1/4 cup wine. Cook, stirring often and scraping up any browned bits from the bottom of the skillet. Season with salt and pepper. Remove from heat.
Place the roasting pan over 2 stovetop burners. Add the remaining 3/4 cup wine. Bring to a boil over medium-high heat, stirring often and scraping up any browned bits. Add the stock. Bring to a simmer. In a small bowl, whisk the flour and 1/2 cup water until smooth, then whisk into the sauce in the roasting pan. Cook, whisking often, until the gravy thickens, 5 to 8 minutes. Stir in the mushrooms.
Slice the roast. Serve with the mushroom gravy. Garnish with thyme sprigs.