Rub the roast all over with the salt and pepper. In a large ovenproof skillet, warm the olive oil over medium heat. Add the roast and brown, letting the meat sear for 1-2 minutes on each side, including the ends, before turning.
In a small saucepan over low heat, melt the butter, then add the dried mushrooms, rosemary, and garlic. Simmer until fragrant and the mushrooms begin to soften, stirring occasionally, for 10-15 minutes.
Position the roast in the large skillet with the fat facing upward, baste the roast with the seasoned butter, and add a few of the mushrooms and rosemary sprigs from the sauce to the pan. Transfer the pork to the oven and baste every 10-15 minutes with the butter sauce, adding all of the remaining mushrooms and rosemary from the sauce after 20 minutes.
Cook the roast until an internal meat thermometer reads 145 degrees at the thickest part, about 1 hour. Remove from the oven, tent with a piece of foil, and let rest for 10-15 minutes. Cut into ½-inch-thick slices and serve, topping with the pan juices and mushrooms from the pan.