For the lamb: Remove the lamb from the refrigerator 30 minutes before cooking.
Warm 2 tablespoons of the olive oil in a large saute pan over medium-high heat. Add the garlic, rosemary, pepper flakes and walnuts. Toast until fragrant and beginning to brown. Add the chopped spinach and 1/2 teaspoon of the salt, and cook until wilted. Stir in the raisins.
Preheat the oven to 350 degrees F.
Spread the lamb out flat, fat-side down, on a board in front of you. If it is still very thick, gently pound it to an even, 1-inch thickness. Sprinkle with 1/2 teaspoon of the salt and the 1/2 teaspoon pepper. Spread evenly with the mustard; distribute the spinach mixture over the mustard. Starting from one side, roll up the lamb with the filling inside so that it maintains the shape of the leg. Tie with butcher's twine at 2-inch intervals. Season the outside with the remaining 1/2 teaspoon salt.
Wipe out the saute pan and add the remaining tablespoon olive oil. Place the pan over high heat and allow it to get hot. Place the lamb in the pan and sear it on all sides until golden brown all over, about two minutes per side. Transfer the lamb to a rimmed baking sheet or a roasting pan and roast until the internal temperature registers 135 degrees F on an instant-read thermometer, about 1 hour 15 minutes. Remove to a cutting board and rest for at least 15 minutes before slicing.
While the lamb is roasting, prepare the gremolata: In a small saute pan, heat the olive oil over medium-high heat. Add the panko and cook, stirring regularly, until golden brown. Add the parsley, lemon zest, pepper flakes and salt. Toss and set aside to let the flavors mingle.
Serve the sliced lamb with a sprinkling of the breadcrumb gremolata.