Season fish inside and out with salt and pepper. Squeeze the juice of 1 lemon into each cavity. Pack equal portion of basil, garlic, onion and tomato into cavity. Tie a loop around one end of each fish and make a few more loops around the fish, securing the stuffing. Brush fish with oil. Place on a medium-low heat barbecue grate for 5 to 7 minutes then flip over cook other side about 7 minutes more or until skin is crisp and fish is just-cooked throughout. To serve, cut string and place fish on plate.