Preheat a barbecue on high heat. In a blender, blend the oil and chilli on high until the chilli is blended. In a large bowl, toss the prawns with 1 tbs of the chilli oil. Season with salt.
Barbecue the prawns, turning as needed, for 4 mins or until charred on the outside but still tender inside.
Meanwhile, using the large holes of a grater, grate the cut side of the tomatoes into a bowl. Discard the skins.
Heat a heavy frying pan on the barbecue. When the pan is hot, add 1 tbs of the chilli oil, the spring onions and garlic and stir for 2 mins or until the garlic is toasted. Add half the parsley and half the oregano and cook for 30 seconds or until the flavours have blended. Add the tomatoes and lemon juice and cook, stirring, for about 2 mins or until the tomatoes have broken down. Fold in the remaining parsley and oregano. Season with salt and pepper.
In a medium bowl, toss the rocket with the remaining 1 tbs chilli oil. Season with salt. Transfer the prawns to the frying pan with the sauce and crumble the feta over. Serve with rocket and lemon wedges.