Step 1Heat the oven to 350 degrees. Measure the lamb and make sure it will fit in your biggest pot. Also make sure the lamb is securely tied in an even cylinder -- your butcher should be able to do this for you. Pat the outside dry and season generously with 1 tablespoon salt and three-fourths teaspoon pepper. Quarter the fennel bulbs lengthwise but do not remove the cores -- they will hold the quarters together.
Step 2Heat the olive oil over medium-high heat in a large Dutch oven and sear the lamb until it is well browned on all sides, about 20 minutes.
Step 3Remove the lamb to a plate and pour off all but about 1 tablespoon of fat. Return the pan to the heat and add the onion and garlic and cook until fragrant, about 3 minutes. Stir constantly, scraping up the browned bits stuck to the bottom of the pan.
Step 4Add the fennel and potatoes and stir to coat with the onion-flavored oil. Season the vegetables with 1 teaspoon salt and one-fourth teaspoon pepper. Add the white wine and stir to combine, scraping up any remaining brown bits in the pan. Push the potatoes and fennel to the sides of the pan and place the browned lamb in the middle, fat-side up.
Step 5Cover the pan and roast in the oven until the internal temperature of the lamb reaches about 120 degrees. Measure in several places and go by the coolest one. This will take about 45 minutes.
Step 6Remove the lid and return the pan to the oven until the lamb is golden brown and the internal temperature has reached 135, about 15 minutes.
Step 7Remove the pan from the oven and set aside for 10 minutes to let the meat rest. Transfer the meat to a carving board and stir the fennel fronds into the vegetables and juices. Remove the strings from the lamb and slice it about one-fourth-inch thick. Transfer to a serving platter, spoon the vegetables around the outside and spoon some of the pan juices over top. Serve warm.