Place a large roasting tray into the oven to heat it up. When hot remove from oven and add a drizzle of olive oil. Toss potatoes, fennel and garlic onto hot sheet tray and sprinkle with salt and pepper. Place into oven on a lower rack.
Take the boneless leg of lamb and open it up. Season inside with olive oil, kosher salt and freshly ground black pepper. Rub with garlic and thyme then make a cut down the middle, separating the lamb into 2 pieces where the muscles naturally separate. Roll each piece up into a small roast and secure with kitchen twine. Rub the outside with olive oil and season with porcini powder, salt and pepper.
Set a large pan over high heat and add a 2-count of olive oil. Sear meat, turning the lamb to brown evenly all over. Once browned transfer the roasts to the oven and place directly on a rack above the fennel and potatoes.
Roast for 35 to 40 minutes until lamb is medium-rare. Rest meat for 10 minutes before cutting. Serve with roasted potatoes and fennel.