Wash the tuna steaks and pat dry with absorbent kitchen paper. Season with salt and plenty of pepper then dust the steaks lightly with flour.
Heat half of the olive oil in a large shallow frying pan and sauté the tuna steaks until golden on one side. Flip them over and cook the other side until golden. Carefully remove them from the pan and transfer to a dish and keep warm.
Heat the remaining oil in the pan, add the onion and garlic and sauté for about 3 minutes until soft and golden. Add the tomatoes, parsley, basil, bay leaf and mashed anchovies and stir well. Heat through gently, allowing the tomatoes to keep their shape.
Return the tuna to the pan, season to taste with salt and pepper and simmer gently for 15 minutes, turning once. Turn off the heat. Add the olives and leave to stand for 5 minutes.
To serve, discard the bay leaf and serve the tuna steaks on the bed of vegetables.