How to Make It
In a bowl, combine the tomatoes, cucumber, olives, onion, lemon juice and the 1 tablespoon olive oil.
Heat a grill pan or preheat the broiler. Brush the eggplant slices with some of the olive oil and season with salt and pepper. Working in batches, grill the eggplant over moderate heat, or broil until very tender and browned, about 2 minutes per side. Transfer the eggplant to a plate and keep warm.
Wipe out the grill pan. Brush the tuna on both sides with olive oil and season well with salt and pepper, pressing the seasoning into the fish. Grill the tuna in the pan over high heat, or broil until nicely browned, about 2 minutes per side.
In a food processor, combine the tahini, garlic and 1/2 cup of water and pulse once or twice to blend. Add the lemon juice, salt and pepper and pulse just until combined.
Make a pool of the Tahini Sauce on each of 6 plates and fan the eggplant slices out on it. Season the tomato salad with salt and pepper and spoon it on top of the eggplant. Set the tuna on the salad and garnish with the chives.