Make your dressing. Combine the juice of 1 lemon and the rest of the ingredients in a small bowl and whisk together. Season to taste with salt and pepper.
Cut the plums and remove the pits. Cut into slices.
Place the arugula in a bowl.
Place the plum slices across the lettuce.
Tear each slice of prosciutto into approximately 4 pieces. Scatter the ham across the salad.