For the tandoori prawns, in a bowl, mix all of the tandoori prawns ingredients, except for the prawns and lemon, until well combined. Add the prawns and mix well until completely coated in the marinade. Cover and chill in the fridge for at least 20 minutes, and preferably overnight.
When the prawns have marinated and 30 minutes before you wish to eat, prepare your barbecue. Allow the flames to flare up and die down until the coals are just glowing.
Meanwhile, for the butter sauce, melt the butter in a frying pan over a medium heat. Add the red onion and fry for 1-2 minutes, or until softened. Add the garlic, cumin and chilli and fry for a further 1-2 minutes.
Add the tomatoes, cream and ginger and bring the mixture to the boil, then reduce the heat until the mixture is simmering and simmer for 4-5 minutes, or until the sauce has thickened slightly and the tomatoes have broken down. Stir in the coriander and season, to taste, with salt and freshly ground black pepper. Keep warm.
Remove the prawns from the bowl, shaking off any excess marinade, and thread onto four metal skewers. Place the skewers onto the barbecue and grill for 2-3 minutes on each side, or until cooked through.
To serve, place the skewers onto serving plates with a lemon half. Place a dollop of the butter sauce alongside each.