1. In a large pestle and mortar, mash the garlic to a paste.
2. Add the yogurt, olive oil, lemon juice, oregano, thyme and salt and pepper.
3. Place the butterflied lamb in a tray and pour the marinade over the lamb, get your hands in there and massage the marinade into the lamb.
4. Cover and refrigerate the lamb for at least 3 hours, or overnight.
5. Remove the meat from the fridge 1 hour before cooking, heat your grill.
6. Scrape any excess marinade off the lamb, then place on the grill. Sear the outside (5 minutes per side) then lower the heat and cook for a further 30 minutes for a medium rare.
7. Rest the meat for 20 minutes then serve with tzatziki and flat breads, couscous or even new potatoes.
8. While the meat is resting make the Tzatziki: Grate cucumber, and sprinkle with 2 tsp salt (this will help extract the moisture).
9. Squeeze the grated cucumber to remove moisture and discard the liquid.
10. In a large bowl, combine cucumber, yoghurt, (these should be approximately equal volume), garlic, oil, red wine vinegar.
11. Cover and refrigerate till serving time.
12. Serve lamb with flat bread and a good dollop of Tzatziki.