Place burghul in a large heatproof bowl and add 2 cups boiling water. Let stand 45 minutes, or until grains are tender and water has been absorbed.
Meanwhile, place fish fillets in a large pan, add lemon slices, parsley sprigs and peppercorns and cold water to cover. Bring to a boil. Reduce heat and simmer, covered, 5 minutes, or until fish is opaque and flakes easily.
Remove fish from liquid and set aside to cool. Use a fork to separate fish into large flakes.
Place burghul in a serving bowl and add cucumber, spring onions, tomatoes and chopped herbs. Gently mix in fish, taking care not to break it up.
To make lemon dressing, whisk all ingredients in a bowl. Pour dressing over salad and mix gently to combine.
Chill salad, covered, 1 to 2 hours to allow flavours to develop. Check seasoning before serving and garnish with mint sprigs.