How to Make It
Light a grill or set a heavy grill pan over moderately high heat. On a work surface, using a fork, mash the garlic with 1/4 teaspoon salt. In a small bowl, blend the garlic with the mayonnaise, basil, chives, 1/4 teaspoon black pepper and the cayenne.
Season the tuna steaks with salt and black pepper and lightly oil the grill or the pan. Grill the tuna for about 2 1/2 minutes, then turn the steaks and top each one with a tomato slice. Grill for 2 to 3 minutes for medium-rare, longer if desired. Transfer to plates.
While the fish is cooking, beat the cream until stiff and fold in the herbed mayonnaise. Serve the fish warm or at room temperature and pass the sauce separately.