First make the stuffing. Place a wok or frying pan over a medium heat and add 2 tablespoons of oil. Sweat the shallots, garlic and chilli for 2-3 minutes, to soften. Add the mushrooms and, when very hot, stir-fry them quickly until browned. Deglaze the pan with rice vinegar and let it reduce until nearly evaporated. Add the pork mince and pepper and fry. Using a wooden spoon, break down the mince. Add 4 tablespoons of water to the stuffing. Pour in the chicken stock with the fish sauce and palm sugar and simmer for 10 minutes. Stir in the chopped coriander for the last 1-2 minutes of cooking.
If your squid has tentacles, finely chop them and add to the stuffing. Simmer, then tip the stuffing into a bowl. Carefully stuff the mixture into each squid, using a toothpick to secure the ends. Repeat the process until all of the squid have been stuffed.
To make the nuoc cham. In a small mixing bowl, dissolve the sugar in the lime juice. Add the remaining ingredients and stir to combine. Serve immediately or refrigerate, covered, for up to 3 days.
Heat 2 tablespoons of oil in a frying pan over a medium heat, add the squid and fry for 30 seconds-1 minute on each side for the baby squid, and 1-2 minutes on each side for the medium-sized squid. Carefully remove the squid from the pan and take out the toothpicks. Slice the squid into thirds or serve the baby squid whole, with the cooking juices poured over. Serve immediately with nuoc cham.