PreparationFor tapenade: Blend all ingredients in mini processor to coarse puree. Season tapenade to taste with salt and pepper. Transfer to small bowl. DO AHEAD: Can be made 3 days ahead. Cover and chill. For burgers: Gently mix diced tuna and next 6 ingredients in large bowl. Using moistened hands, shape tuna mixture into six 3/4-inch-thick patties (each about 4 inches in diameter). Place burgers on plate. Cover and refrigerate at least 1 hour and up to 1 day. Spray grill rack with nonstick spray. Prepare barbecue (medium-high heat). Grill burgers until almost cooked through, about 4 minutes per side. Arrange 6 focaccia squares or bottoms of pain rustique on work surface; spread with tapenade. Top each with burger, tomato slice, onion slice, and arugula, then focaccia or bread top. Cut burgers in half and serve. Test kitchen tip:Chilling the tuna will make it easier to dice. Keep the formed patties in the fridge until you're ready to cook them. This will prevent the burgers from falling apart on the grill.
PreparationFor tapenade: Blend all ingredients in mini processor to coarse puree. Season tapenade to taste with salt and pepper. Transfer to small bowl. DO AHEAD: Can be made 3 days ahead. Cover and chill. For burgers: Gently mix diced tuna and next 6 ingredients in large bowl. Using moistened hands, shape tuna mixture into six 3/4-inch-thick patties (each about 4 inches in diameter). Place burgers on plate. Cover and refrigerate at least 1 hour and up to 1 day. Spray grill rack with nonstick spray. Prepare barbecue (medium-high heat). Grill burgers until almost cooked through, about 4 minutes per side. Arrange 6 focaccia squares or bottoms of pain rustique on work surface; spread with tapenade. Top each with burger, tomato slice, onion slice, and arugula, then focaccia or bread top. Cut burgers in half and serve. Test kitchen tip:Chilling the tuna will make it easier to dice. Keep the formed patties in the fridge until you're ready to cook them. This will prevent the burgers from falling apart on the grill.