For Pork Belly, Preheat oven to 300°. Season pork belly with salt and the five spice powder using a touch of oil to coat. Create a pouch using aluminum foil. Lay sliced onion and garlic on the bottom followed by pork. Seal the pouch seam side up and roast on baking sheet until tender, 2 to 2 1/2 hours (you want the belly to be tender when punctured with a knife but not falling apart). Cool in pouch overnight in the refrigerator so the belly is firm enough to cut.
For Caramelized Miso-Sesame Mayo, preheat oven to 400 °. Spread miso on a silpat lined rimmed baking sheet and bake until golden, 8 to 10 minutes. Place cooked miso in a blender along with brown sugar and a few tablespoons of water. Blend on high until smooth. Add mixture to a bowl and whisk in Hellmann's® or Best Foods® Real Mayonnaise and sesame oil.
When ready to serve, slice chilled pork belly across into 1/2-inch thick slabs (they should look like thick bacon pieces), then slice each slice in half, or thirds if the belly is large. Heat a wok and sear pork belly on both sides, allowing to caramelize fully. Set aside.
For Assembly, open buns and smear with Caramelized Miso-Sesame Mayo. Lay cucumber slices on the bottom, top with warm pork slices, scallions, and a squirt of Sriracha, chili sauce and sesame seeds (optional). Serve warm.
*No filling steamed buns can be found in the frozen section of an Asian supermarket.