Cook the squares of pork belly in a pan of boiling water for five minutes, then drain, pat dry with kitchen paper and set aside to cool.
Heat the vegetable oil in a large frying pan over a medium to high heat. Add the squares of boiled pork belly, skin-side down, in batches, and fry for 6-8 minutes, or until crisp and golden-brown. Turn the pork belly squares over and fry for a further 3-4 minutes, or until crisp. Remove from the pan using a slotted spoon and set aside to drain on kitchen paper. Repeat the process with the remaining batches of pork belly.
Bring all of the remaining ingredients to the boil in a heavy-based saucepan. When the mixture is boiling, add the fried pork belly and reduce the heat until the mixture is simmering. Continue to simmer the mixture for 45-50 minutes, adding a little of the chicken stock to the pan, as necessary, if the mixture starts to dry out.
Meanwhile, for the mashed potatoes, boil the potatoes in a pan of salted water for 15-20 minutes, or until tender. Drain well, then mash until smooth using a potato masher or ricer.
Stir the butter and saffron-infused milk into the mashed potatoes until well combined, then season, to taste, with salt and freshly ground black pepper. Stir in the spring onions.
To serve, spoon a portion of the spring onion mash into the centre of each of four serving plates. Place two to three squares of pork belly on top of each serving, then spoon over the sauce.
