In a medium bowl, combine the soy sauce, mirin, brown sugar, sesame oil, garlic, ginger and red pepper flakes. Whisk to combine. Place the pork belly in a plastic or glass container and pour half of the marinade over it. Turn the pork belly several times to coat. Cover and refrigerate for 3-4 hours or overnight.
Pour the remaining marinade into a small pan and heat over medium high heat, whisking constantly until mixture comes to a boil. Reduce heat and simmer 3-5 minutes until slightly reduced. Let the mixture come to room temperature. Transfer to a storage container and refrigerate until ready to serve.
After pork has marinated, preheat oven to 300°. Lay two large pieces of tin foil on top of each other and transfer the pork belly (not the marinade) to the center of the tin foil. Seal it up very tightly, to keep the juices in. Place the foil packet on a baking sheet and cook for 2 1/2 hours. Open up the foil packet to expose the pork, but still holding the juices and turn the oven up to 450°. Cook the pork for 20 minutes until golden. Remove from oven and cool to room temperature. Reseal the foil package and refrigerate several hours or overnight.
Transfer the pork belly to a cutting board and slice crosswise into 1/4" strips.
Working in batches, add 6-7 strips of pork to a large skillet, along with a little fat and drippings. Cook over medium high heat until fragrant and golden. Add 3-4 tablespoons of reserved marinade and cook for 2-3 more minutes to glaze the pork.
Serve pork belly in Asian Steamed Buns with carrots, cucumber, red chili, green onion, sesame seeds and a drizzle of reserved marinade.