To make the tzatsiki, chop the cucumber into small cubes, place in a sieve and allow to drain. Smash the garlic into a paste with the flat of a knife.
In a large bowl, mix the garlic, a good pinch of salt and a third of the chopped mint.
Add a squeeze of lemon juice and the olive oil, mix well and then add the yoghurt, followed by the drained cucumber. Mix gently and chill in the fridge.
Trim any sinew from the lamb fillets. Heat two tablespoons of oil in a large pot and seal the lamb. Remove the fillets from the pot and place to one side.
Add the garlic to the pot and quickly add the balsamic vinegar and red wine. Bring to the boil, then add the rosemary and half the remaining chopped mint. Now add the meat stock and sugar with salt and pepper to taste and allow to simmer until reduced by half.
While it simmers, dust the lamb fillets with celery salt and dried oregano and place in the simmering juices. Cover with a lid and cook gently for five minutes.
Remove the lamb fillets and reduce the sauce by half, squeezing in the lemon juice.
To prepare the couscous, soak it as per the packet instructions, then add the toasted pine nuts, raisins, chopped parsley, the remaining chopped mint, the lemon juice and zest, olive oil, cumin and salt and pepper to taste. Mix well.
While the couscous soaks, cook the green beans in boiling salted water for 4-5 minutes until just tender. Drain well.
Slice each lamb fillet into pieces and place over the couscous and green beans. Spoon over the lamb juices and finish with a big dollop of tzatsiki. Serve.