Put a kettle on to boil. Place the bulgur wheat in a large heatproof bowl, cover it with 600 ml (1 pint) of boiling water and leave it to stand for 45 minutes, or until it is tender and has absorbed all or most of the water.
Meanwhile, place the fish fillets in a large lidded frying pan, add the lemon slices, parsley sprigs and peppercorns and enough cold water to cover. Bring it to the boil, then reduce the heat and simmer, covered, for 5 minutes, or until the fish is opaque and flakes easily.
Remove the fish from the liquid and leave it to cool, then break it gently into large flakes.
Drain off any excess water from the bulgur wheat and squeeze it dry. Put it into a large salad bowl and add the cucumber, spring onions, tomatoes and chopped herbs. Gently mix in the fish without breaking it up.
To make the dressing, whisk all the ingredients together. Pour the dressing over the salad and mix gently without breaking up the fish.
Cover the bowl and chill in the refrigerator for 1-2 hours to allow the flavours to develop. Check the seasoning before serving and garnish with extra sprigs of mint.
Variation: you can substitute any firm white fish for the orange roughy, or use canned tuna in water, drained and flaked.