1Heat the oven to 250°F. To a roasting pan or other large ovenproof dish, add the lamb belly, salt, pepper, garlic cloves, fennel, cumin, and paprika. Mix everything together, cover the pan with foil, and cook until the meat softens, about 10 hours.
2Remove the meat from the resulting braising liquid and allow both to cool. Lift the solidified fat from the liquid, pour the liquid into a saucepan over medium-high heat, and reduce until thickened. Shred the meat by hand and recombine with the thickened liquid. Use right away, or cool and store in the fridge.
To assemble the sandwich:
1Toast the rolls and paint them with the melted butter. Spread 2 tablespoons of the harissa aioli on a roll and top with 1/3 pound of the hot pulled lamb, some torn cilantro, wild arugula, and a squeeze of lemon. Repeat with the remaining rolls.
A good Reuben sandwich is a perfect balance of flavors and textures, and this one delivers. Good rye bread, crisped in butter, holds generous layers of Thousand Island dressing, rich corned beef or pastrami, crunchy sauerkraut, and nutty, melty Gruyère or Swiss cheese that oozes beautifully into the other ingredients.