Place the bulgar wheat in a large bowl and cover with hot water, then leave to stand for about 20 minutes to swell up. Tip into a sieve and allow to drain.
Meanwhile, make the dressing. Place the lemon in a screw-topped jar with the garlic and allspice. Season to taste and shake until the salt has dissolved. Add the olive oil and shake again until well combined.
Tip the drained bulgar wheat into a bowl and add the tomatoes, spring onions, parsley, mint and preserved lemon. Pour over the dressing and stir gently until well combined.
Flake the smoked trout fillets into large chunks, discarding any skin and bone and then fold gently through the bulgar wheat mixture, reserving a few nice chunks for the top of each serving.
To serve, divide the smoked trout and tabbouleh salad among serving plates and scatter the reserved trout on top. Mix the Greek yoghurt in a small bowl with the dill and enough water to give the consistency of pouring cream. Season to taste and drizzle on top.