Chop 1 cup pitted green or kalamata olives and transfer to a medium bowl. Stack 6 basil leaves, roll into a tight cylinder and thinly slice. Add to the bowl. Finely chop 2 scallions and 1 garlic clove. Add to the bowl. Add 1/4 cup olive oil and 1/4 cup grated parmesan cheese to the bowl and toss. Arrange 20 to 24 crostini, crackers or baguette slices on a platter. Divide the mixture evenly among the crostini. Drizzle with olive oil and sprinkle with more grated parmesan.