Spoon the mustard into a medium bowl. Whisk in half the oil and half the vinegar until blended. Add remaining oil and vinegar, and whisk until blended. Whisk in water, and season with salt and pepper. Or, use a food processor.
Next, place prosciutto in a small skillet on medium heat and sauté until it begins to brown. Add the minced garlic and brown together for a few seconds. Add vinaigrette and cook just until warm.
Meanwhile, loosen each Romaine heart on 4 individual salad plates. Crumble blue cheese onto the lettuce, the optional hazelnuts, and drizzle with the warmed prosciutto (or Westphalian, better) dressing. Serve.