Process lamb in 2 batches until minced. Transfer to a large bowl. Process onion, mint and parsley until finely chopped. Add to lamb with cumin and coriander. Add couscous and mix until well combined. Season with salt and pepper.
Divide lamb mixture into 8 portions. Using wet hands, shape each portion into a thick sausage around a metal skewer. Put kofta on a plate, cover and refrigerate for 1 hour or till firm.
To make Tomato salsa: Combine tomatoes, mint, parsley, lemon juice, oil, sugar, and salt and pepper in a non-metallic bowl. Cover and stand at room temperature for 30 minutes to allow the flavours to develop.
Preheat a lightly oiled barbecue grill or flat plate on medium until hot. Brush individual kofta with olive oil and then barbecue, turning several times, for 8-10 minutes until cooked through. Serve with Tomato salsa.