Skin the smoked mackerel fillets and break the meat into small flakes. Pour the oil into a frying pan to a depth of 2cm and heat to 190C (or until a small piece of bread browns in 20 secs). Sprinkle the fish pieces into the oil and deep-fry for 1 min or until crispy. It will probably all stick together at this point, but don’t worry. Lift the fish out onto lots of kitchen paper, leave to cool, then break up into pieces again.
Peel the mango and carrot, then shred both (using a mandolin or shredder) into strips. Put the mango, carrot, shallots, chilli, peanuts and fish pieces in a large bowl and toss together. Mix the sugar with the fish sauce and lime juice (you can add more or less than 1 tbsp, depending on the tartness of the mango). Add to the salad with the Thai basil and toss together again. Pile into the centre of two medium-sized plates – or four small plates if serving as a starter – and serve immediately.