Place the mackerel on a baking sheet and use a chef's blowtorch to quickly cook the mackerel (alternatively cook them under a very hot grill for a few minutes).
Bring a large saucepan of water to the boil, then add the pak choi and string beans to blanch for 30 seconds. Drain, then transfer into very cold water to stop the cooking. Drain again and pat dry.
Heat a large griddle pan and drizzle a little oil over the pak choi. Cook on the hot griddle for a couple of minutes on each side until lightly charred. Remove the pak choi and keep warm. Drizzle a little oil over the string beans and griddle them for a few minutes until lightly charred.
For the chilli jam, put all the chilli jam ingredients, except the sugar and crème fraîche, in a food processor and blend to a smooth purée.
Heat the sugar in a heavy-based saucepan over a low to medium heat until it melts and forms a golden-brown caramel. Do not stir and keep an eye on it so that it doesn’t burn.
Once the sugar has caramelised, carefully pour in the puréed chilli jam mixture and stir well (the mixture will bubble up a lot, so take care). Bring the mixture to the boil, then reduce the heat and simmer for 4-5 minutes, or until sticky and jam-like. Remove from the heat and set aside to cool slightly.
To serve, place a fillet of mackerel on one side of the plate and serve the pak choi and beans on the side. Drizzle the chilli jam over the fish and garnish with the coriander cress and a spoon of crème fraîche.