Cut each chicken breast widthways into 4 pieces and place between freezer film. Flatten with a meat mallet until 2–3 mm (1/16–⅛ inch) thick. Remove the freezer film and thinly slice the chicken with a sharp knife — it should tear apart and resemble coarse mince. Place in a bowl and add enough fish sauce to lightly coat. Add the flour and, using your fingers, toss until coated and separated into clumps, then place in a sieve and shake to remove excess flour.
Heat the oil in a large deep saucepan or deep-fryer to 180°C (350°F). Deep-fry the chicken, turning regularly with tongs or shaking the fryer basket, for 1 minute or until golden and crisp. Remove and drain on paper towel. While hot, toss in sumac. Serve immediately.