Tip the bulghar into a pan, cover with water and boil for 10 mins. Drain really well in a sieve, pressing out any excess water. To make the tzatziki, squeeze and discard the juice from the cucumber, then mix into the yogurt with the chopped mint and a little salt. Work the bulghar into the lamb with the spices, garlic (if using) and seasoning, then shape into 4 burgers. Brush with a little oil and fry or barbecue for about 5 mins each side until cooked all the way through. Serve in the buns (toasted if you like) with the tzatziki, tomatoes, onion and a few mint leaves.