1. Whisk together the olive oil, vinegar and mustard and season well.
2. Trim the root and separate the leaves of the lettuce; slice the hearts in two. Place in a large bowl with the arugula, and pour over the dressing. Mix well; pile on a serving plate.
3. Arrange the melon cubes over the salad and top with the cheese and the prosciutto slices, torn into strips. Scatter the parsley on top and serve.