Preheat a grill or BBQ pan and brush with oil.
Add the basil, rocket, lemon juice and zest, garlic, oil and seasoning to a blender and blend until all the ingredients are well combined.
Score the mackerel along the sides of its thickest parts of its body and rub in the basil and rocket paste both inside and out.
Sprinkle the fish with the herbs and tomatoes and transfer to the grill.
Cook for 25 - 30 minutes turning once after 15 minutes.
Shopping Tip: Make the fish shop the last port of call before going home. Transport the mackerel in an esky or cool bag and refrigerate it immediately in coldest part of your fridge. Cook on the night of purchase.