Put the tuna steaks into a shallow dish, mix the olive oil and lemon juice together with a little seasoning, pour over the steaks and leave to marinate for 15 minutes while making the sauce.
Put all the ingredients for the sauce into a blender or food processor and blend together, transfer to a pan and heat gently.
Place the tuna on a pre-heated barbecue, hot enough to sear the fish for 3 minutes until the fish begins to mark. Use a large fish slice to turn the steak and cook for a further 2-3 minutes until well-marked (this will depend on the thickness of the steaks).