Cut the fish into four equal-sized pieces. Season and dust with 2 tsp of the flour. In a medium bowl, whisk the remaining flour, egg, coconut milk and baking powder with some salt until you have a smooth, thick batter. Add a little more coconut milk if your mixture is particularly stiff.
In a small bowl, mix together the tartare sauce ingredients and set aside.
Pour vegetable oil into a large wok to a depth of 2cm and heat – it’s hot enough when a small piece of bread browns in 20 secs. Fry the fish in batches, dipping each piece into the batter, then gently lowering into the oil. Fry for 2-3 mins until golden, turning once. Drain on kitchen paper, then put in a low oven to keep warm while you fry the rest.
Serve the fish with the chips, peas, tartare and lemon wedges.