Remove the skin from the tuna steaks, then cut the steaks in half along the grain of the fish.
Cut each half into 1cm / 1/2in thick slices against the grain.
Put 3 tablespoons of the olive oil and the vinegar in a large, shallow, non-metallic dish. Strip
the leaves from the sprigs of thyme, add these to the dish with the bay leaf, and season with salt
and pepper. Add the prepared strips of tuna, cover the dish and marinate in the refrigerator
Put the flour in a plastic bag. Remove the tuna strips from the marinade, reserving the marinade
for later, add them to the bag of flour and toss well until lightly coated.
Heat the remaining olive oil in a large frying pan. Add the onion and garlic and cook gently for
5-10 minutes, or until softened and golden brown. Add the tuna strips and cook for 2-5 minutes,
turning several times, until the fish becomes opaque. Add the reserved marinade and olives and cook
for a further 1-2 minutes, stirring, until the fish is tender and the sauce has thickened.
Serve the tuna and olives piping hot, garnished with thyme sprigs.