For the marinated pork belly: Put the pork belly, garlic and enough cold water to cover by 2 inches in a pot and bring it to a boil over high heat. Turn it down to simmer and cover three-quarters of the way so a little steam escapes. Cook for 90 minutes, then take off the stove. Strain the liquid from the pork belly and reserve the pork belly in the pot.
For the tare: Put soy sauce, mirin, sugar, dark soy sauce and 4 cups water in a pot and bring it to a soft boil over high heat, stirring. Let cool to room temperature.
Pour tare over the pork belly and make sure it completely covers or submerges the pork belly (weigh it down with a plate if necessary). Leave the pork belly in the tare, turning occasionally, for 1 hour at room temperature. Remove the pork belly from the tare to a sheet pan or a plate and refrigerate until firm, about 2 hours.
For the eel sauce: Combine soy sauce, sugar, mirin, ginger, garlic, onion and 1/2 cup water in a pot over high heat, stirring. Right before it starts to boil, turn the heat down to medium-low. Simmer, uncovered, stirring every 30 minutes or so, until it reduces by about half, about 2 hours. Strain the liquid from the veggies and aromatics and cool for about 1 hour in the fridge.
For the nigiri: Make an oval ball of rice about 2-by-1-inch. Slice the marinated pork belly 1/3-inch thick. Put a slice of pork belly on top of the rice and gently mold them together so they lightly stick. Wrap a strip of nori around the rice and the pork belly to secure them together. Put on a heat-resistant plate and gently pour about 2 teaspoons eel sauce on top of the pork belly. Use a kitchen torch to heat the pork belly until the eel sauce caramelizes. Top with Pickled Mustard Seeds, green onions, chives and leeks, and finish with a drizzle of chile oil. Repeat with remaining ingredients.
Pickled Mustard Seeds:
Stir together the vinegar, mustard seeds and 1 cup water. Let the seeds absorb the liquid, refrigerated in an airtight container, 2 days.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.
Propane gas torches are highly flammable and should be kept away from heat, open flame and prolonged exposure to sunlight. They should be used only in well-ventilated areas. Follow torch manufacturer's instructions for use.