Preheat An Outdoor Grill For Medium-High Heat And Lightly Oil Grate. Stir Together Grapes, Capers, Shallot, Parsley, And Olive Oil In A Bowl; Season To Taste With Salt And Pepper, And Set Aside. Place Tuna Steaks Onto A Plate, And Brush With Lemon Juice. Season With Salt And Pepper To Taste. Cook Tuna Steaks On Preheated Grill Until Cooked To Desired Degree Of Doneness, 2 To 3 Minutes Per Side For Medium-Rare. Serve With The Grape And Caper Salsa.