1. Combine first 5 ingredients in a food processor; process until smooth.
2. Place 1 1/2 cups arugula on each of 8 salad plates. Top each serving with 3 honeydew slices, 3 cantaloupe slices, and 1 prosciutto slice. Drizzle each serving with about 1 tablespoon dressing; sprinkle evenly with pepper.