Preheat barbecue for medium-high heat and lightly oil grate.
Stir together grapes, capers, eschalot, 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 barbecue until cooked to desired degree of doneness, 2 to 3 minutes per side for medium-rare. Serve with the grape and caper salsa.