Combine beef mince, spice mix, mint, egg yolk and breadcrumbs in a large bowl. Season with salt and pepper. Using your hand bring the mixture together and knead until just combined. Shape mixture into 12 x 12cm long sausage shapes and thread onto 12 greased metal skewers or 12 soaked bamboo skewers. Transfer to a plate and chill for 20 minutes to set firm.
Meanwhile, place the couscous and ¼ cup boiling water into a heatproof bowl. Cover and stand for 5 minutes or until the couscous has absorbed the water. Using a fork, fluff to separate the grains. Stir the parsley, green onion, oil, tomato and extra mint leaves into the couscous until well combined. Season with salt and pepper.
Using a sharp knife slice the cheeks off the lemons. Set the cheeks aside. Squeeze the remaining lemon over the couscous tabouli.
Preheat a char-grill to medium. Cook beef kebabs for 6-8 minutes, turning occasionally, or until golden and just cooked. Adding the lemon cheeks, flesh side down, to grill plate in the last 2 minutes of cooking. Transfer beef kebabs and lemon cheeks to serving plates. Serve with couscous tabouli.
If using bamboo skewers for the kebabs, soak them in water for at least 30 minutes before using as this will prevent them from burning when grilling.
Substitute long grain white rice instead of couscous.