To prepare the garlic, finely mince the garlic cloves and set aside.
To prepare the pork belly, use a fork to pierce the pork belly pieces all over to help them cook faster and absorb flavor.
To make the sauce, in a bowl, combine the mirin, sake, soy sauce, dark soy sauce, sugar, and water, then whisk until the sugar dissolves.
To sear the pork belly, in a pan over high heat, add the pork belly pieces and cook until golden on both sides, about 2 minutes per side. Add the minced garlic and stir-fry for about 10 seconds until fragrant.
To simmer the pork belly, pour the sauce into the pan, reduce the heat to medium, cover, and cook for about 15 minutes, flipping the pork once halfway through.
To finish the dish, uncover the pan and increase the heat to high. Reduce the sauce for about 5 minutes, spooning the sauce over the pork until it thickens into a glossy glaze. Slice into bite-sized pieces and serve with rice or noodles.