Photograph: Dan Jones
Preheat the oven to 150°C, fan 130°C, gas 2. Tip the tempura batter mix into a bowl and add the chilli flakes. Whisk in 150ml of cold water to make a smooth batter.
Season the fish pieces, add to the batter and turn to coat. In a deep pan, heat about 5cm of oil to 180°C, or when a small cube of bread turns golden in 60 seconds. Fry the fish in batches for about 4 minutes or until crisp, turning occasionally. Set aside to drain on kitchen paper while you cook the remaining fish. Meanwhile, wrap the tortillas in foil and warm in the oven for 10 minutes.
Divide the fish, avocado and tomatoes among the tortillas. Add a few sprigs of coriander, a pinch of chilli flakes, a squeeze of lime and a dollop of soured cream before serving.