Combine salt with sugar, and hot water, stirring until the salt and sugar have dissolved. Let cool, add the pork belly and refrigerate overnight.
Preheat oven to 325 degrees F.
Drain the pork belly and pat dry. Combine the coriander with garlic powder, fennel, cumin, and black pepper in a small bowl. Rub the spice mixture onto the pork belly, covering all sides except for the skin.
Heat 2 tablespoon oil in a Dutch oven or braising pot over medium heat. Lightly sear the pork until golden brown on all sides. Add the white wine and chicken broth and bring to a boil. Cover the pot and continue cooking the pork in the oven for 2 hours or until the meat is very tender. Remove from heat and let rest for 30 minutes. Slice the pork belly.
Meanwhile, prepare the Meyer lemon relish. Combine all the ingredients in a medium bowl. Season with salt and pepper and let sit for 10 minutes.
Prepare the mustard aioli. Mix stone ground mustard with mayo in a small bowl until well combined. Set aside.
Assemble the sandwiches. Smear each bottom bun with mustard aioli and top with arugula, sliced tomatoes, pork belly, and the Meyer lemon relish. Serve immediately.