Heat a tiny drizzle of oil in a frying pan and add the prosciutto. It will start to crisp up immediately. Fry for about 30 second, flipping halfway through then transfer to a paper towel lined plate.
Butter one side of each slice of bread and fry in the same pan until browned and crispy. Remove to a cutting board. Fry your eggs to your liking.
Spread a layer of mayo and a layer of mustard on the unfried side of two of the slices of bread. Top with 3 slices of prosciutto each followed by half the watercress, an egg and the other slice of bread (fried side out). Serve.
Nutrition information is an estimate and will vary depending on the exact ingredients used.