Bring a pan of well-salted water to a gentle simmer. Add the pork belly and simmer for 15 to 20 minutes, keeping the water below a rolling boil. A hard boil will tighten the meat and toughen the outside before the inside has had time to relax. After 15 to 20 minutes the pork should be firm to the touch but not cooked through. Remove, leave to cool slightly, then cut into 2cm by 2cm chunks.
In a separate pan, bring salted water to the boil and blanch the pumpkin chunks for 6 to 8 minutes until just starting to soften but still holding their shape. They should take a little resistance when pressed. Refresh immediately under cold running water to stop the cooking and set the colour. Drain and set aside.
Toss the blanched pumpkin in the 5g of coarse sea salt while still warm and leave it to sit for 10 minutes. The salt draws out a little moisture and concentrates the flavour before it goes into the sauce.
In a wide, heavy-bottomed ovenproof pan, combine the palm sugar, vegetable stock, oyster sauce, dark soy sauce, pandan leaf, cassia bark, star anise and 50ml of the fish sauce. Place over a low heat and stir until the sugar has completely dissolved. Don't rush this: high heat will cause the sugar to catch on the base before it's had time to incorporate with the liquid.
Add the blanched pork belly pieces and stir to coat in the sauce. Lay a circle of greaseproof paper (a cartouche) directly onto the surface of the liquid and cover the pan. Bring to a very gentle simmer and braise for 30 to 40 minutes, turning the pork occasionally, until the meat is tender and beginning to collapse at the edges and the sauce has thickened and darkened considerably.
Add the salted pumpkin and fold in gently, being careful not to break the pieces apart. Simmer uncovered for a further 5 to 10 minutes until the pumpkin is soft and the sauce has reduced to a thick, glossy, dark braising liquor that coats everything heavily. Taste now: the sauce should be deeply sweet from the palm sugar, savoury and slightly bitter from the dark soy and oyster sauce, with a rich saltiness underneath. If it needs more seasoning, add the remaining fish sauce a little at a time. If it tastes one-dimensional and flat, a splash more fish sauce will bring it into balance immediately.
While the pork finishes, heat the sunflower oil in a small saucepan to 180°C. Test with a single ring of shallot: it should sink slightly, then rise and sizzle steadily. Add the sliced banana shallots in batches and fry for 2 to 3 minutes until golden and crisp. Lift out with a slotted spoon onto kitchen paper. Separate the rings with a fork while they're still hot: they'll crisp further as they cool and stick together if left in a pile.
Serve the pork and pumpkin directly from the pan, scattered with the crispy shallots and sliced spring onions. Sticky rice alongside to soak up the braising liquid. Som tam if you have it.