Preheat the oven to 350 degrees. In a mixing bowl, toss the bread slices with the olive oil, coating each side completely. Season the bread with salt and pepper. Place the bread on a baking sheet and bake until crispy and slightly golden brown, 6 to 8 minutes. Remove the croutons from the oven and place on a large plate. Season the tuna steak with olive oil, salt and pepper. Place the tuna on a hot grill and cook for about 2 to 3 minutes on each side for medium-rare. Combine all of the remaining ingredients in a food processor and puree for 15 seconds or until the mixture is smooth. Slice the tuna into 12 equal slices and place one slice on top of each crouton. Dab a spoonful of the tapenade on top of each tuna slice. Garnish with chopped chives.
Yield: 1 dozen crostinis