In a food processor combine cilantro, mint, lime juice, jalapeno pepper, and garlic. Cover and process until finely chopped. (If you do not have a food processor, finely chop the herbs, jalapeno, and garlic; combine with the lime juice.) Stir in Parmesan cheese.
If desired, remove crusts from bread. Spread one side of each bread slice with softened butter. Top 2 of the bread slices with half of the cheese. Top with all of the cucumber slices. Carefully top with all of the cilantro mixture, remaining cheese slices, and remaining bread slices, buttered side down. Cut sandwiches in half. Serve immediately or wrap in plastic wrap and pack with an ice pack or chill up to 4 hours before serving. Makes 2 sandwiches.