How to Make It
In a large nonstick frying pan, heat 1 tablespoon of the oil over moderately high heat. Sprinkle 1/4 teaspoon each of the salt and the pepper over the tuna. Cook the fish for 2 minutes. Turn and cook until done to your taste, 2 to 3 minutes longer for medium rare. Transfer the tuna to a carving board.
In a small glass or stainless-steel bowl, whisk together the lemon juice, garlic, and the remaining 1/4 teaspoon salt and 1/2 teaspoon pepper. Add the remaining 6 tablespoons oil slowly, whisking.
Cut the tuna into 1/2-inch-thick slices. In a large glass or stainless-steel bowl, combine the lettuce and the radishes and toss with all but 2 tablespoons of the vinaigrette.
Put the salad on plates. Top with the tuna slices and drizzle the remaining vinaigrette over the top.
Substitute two six-ounce cans of drained canned tuna for the fresh tuna. Skip Step 1 completely and add the drained canned tuna along with the lettuce and radishes.