For the pork belly: Line a baking sheet with foil and fit with a wire rack.
Mix together the olive oil, thyme, mustard powder, coriander, red pepper flakes, garlic, 2 teaspoons salt and a generous amount of pepper in a small bowl until combined. Mix the baking powder with 2 teaspoons salt in a separate small bowl.
Pat the skin of the pork belly completely dry. Use a sharp skewer, fork or paring knife to poke lots of holes all over the skin, making sure not to hit the meat. This will help the skin crisp up. Flip the pork over and rub the spice mixture all over the meat (not the skin). Transfer to the prepared baking sheet skin-side up. Sprinkle half of the baking powder mixture over the skin. Refrigerate, uncovered, for at least 1 hour and up to 6 hours.
Preheat the oven to 325 degrees F.
Wipe off the baking powder mixture from the pork with a paper towel. Pat the skin completely dry and sprinkle with the remaining baking powder mixture.
Bake until the pork is cooked through and tender, about 2 hours. Raise the temperature to 500 degrees F and continue cooking until the skin is golden brown, bubbling and crispy, about 20 minutes more. Let rest for 10 minutes.
For the sauce: Meanwhile, whisk together the cilantro, olive oil, honey, scallions, garlic, lemon juice, 1/2 teaspoon salt and a few grinds of pepper in a medium bowl.
Slice the pork belly into 1/2-inch-thick slices and transfer to a serving platter. Spoon the sauce down the middle before serving.
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