A day or two before serving, prepare the pork belly. Bring the pork belly, cinnamon stick, star anise, half the ginger, and enough water to cover by several inches to a boil in a medium saucepan over high heat. Reduce the heat and simmer for 30 minutes.
Drain the pork, discarding the liquid and spices. Marinate the pork with the green onions, black soy sauce, and the remaining ginger for 2 hours.
Remove the pork, discarding the marinade. Pat the pork dry with paper towels and allow it to air-dry on a rack or sieve for 1 hour. Heat 2 inches of oil to 250°F in a wok over low heat. Add the pork and deep-fry, flipping occasionally, until the skin is just beginning to crisp, 7 to 10 minutes. Drain.
Using a Thai-style steamer, steam the pork until fully cooked, about 45 minutes. When cool enough to handle, slice the pork belly lengthwise as thinly as possible.
Crack the black cardamom pods, discarding the shells. Wipe out the wok, set over low heat, and add the black cardamom seeds. Toast until slightly dark and fragrant, about 10 minutes. Grind the seeds to a fine powder with a mortar and pestle or a spice grinder. At least 5 hours before serving, bring 2 cups of water to a simmer in a medium saucepan. Remove from the heat, add the oolong tea, and steep for 3 minutes. Strain, retaining the liquid and the tea leaves separately. Chop the tea leaves. Return the tea water to the saucepan over high heat, add the tea leaves, pickled greens, white pepper, 1 teaspoon of the black cardamom powder, and the white soy sauce. Bring it to a boil, reduce the heat, and simmer until nearly all the liquid has evaporated, approximately 45 minutes.
Taste, adjusting the seasoning if necessary; the greens should be salty, spicy, and slightly astringent.
Neatly line the inside of a medium ceramic or glass bowl with the slices of pork. Top with the mustard green mixture. Add the dish to a steamer over high heat. Bring the water to a boil, reduce to low, and gently steam for 4 hours. Add water to the steamer as necessary so you don’t scorch the pot.
To serve, invert the bowl on a serving plate. Garnish with the cilantro and serve warm or at room temperature, with mantou or long-grained rice, as part of a Chinese meal.