Brush the lamb loins with the oil and rub with the oregano and some salt and pepper. Set aside to marinate for 1 hour.
Meanwhile, place the beetroot in a saucepan of cold water, bring to the boil and cook for 20 minutes until just tender. Drain well, cut into wedges and place in a bowl. Add a little oil and thread on to 2 metal skewers. Cook on a hot barbecue for 6—8 minutes on each side until lightly charred. Return to the bowl.
Blanch the beans in lightly salted boiling water for 3 minutes. Drain, refresh under cold water and dry well. Add to the beetroot with the feta, spinach and walnuts.
Whisk the dressing ingredients and some salt and pepper together in a small bowl.
Cook the lamb on a hot barbecue for 4—5 minutes on each side, then set aside to rest briefly. Serve in slices with the salad, drizzled with the dressing.