How to Make It
In a large pot, combine the salt, sugar, bay leaves and boiling water; stir to dissolve the salt and sugar. Add the ice water and let cool. Add the pork belly and refrigerate overnight.
Drain the pork belly and pat dry with paper towels. In a small skillet, combine the coriander seeds, fennel seeds, peppercorns, cumin, clove and allspice and toast over moderate heat until fragrant, about 3 minutes; transfer to a spice grinder and let cool completely. Coarsely grind the spices and rub them all over the pork belly. Transfer to a platter, cover and refrigerate overnight.
Preheat the oven to 325°. Rinse the pork, scraping off most of the spices, and pat dry. Heat a large, deep ovenproof skillet until hot. Add the pork, fatty side down, and cook over moderately high heat, turning once, until browned on both sides, 8 minutes. Scatter the garlic, onion, celery and carrots on either side of the pork belly and cook until softened slightly, 5 minutes. Add the wine and boil until reduced to 1/4 cup, 5 minutes. Add the broth and bring to a boil. Cover and braise in the oven for 2 hours, until the meat is very tender but not falling apart. Transfer the pork to a platter and refrigerate for 30 minutes. Cut the meat into 1/2-inch-thick slices, then cut each slice in half so it will fit on the ciabatta.
In a large nonstick skillet, fry the pork slices in batches over moderately high heat, turning once, until crisp, about 4 minutes.
Spread mustard on the roll bottoms and top with the sliced pork belly, frisée and a generous dollop of Meyer Lemon Relish. Close the sandwiches, cut in half and serve.