Trim any skin or dark or bloody spots off the tuna. Rinse the tuna under cold running water and blot dry with paper towels. Arrange the steaks in a nonreactive baking dish.
Combine the basil, garlic, lemon zest, lemon juice, vinegar, oil, salt, and pepper in a food processor or blender and purée until smooth. Pour this mixture over the tuna and let marinate in