1. One by one, place the chicken breasts between 2 sheets of Saran Wrap, smooth side down, and with the even side of a meat hammer (or with a cast iron pan), flatten them to approximately 1/4 inch thick. Remove Saran Wrap once all chicken breasts are done, and apply a thin layer of the mustard on 1 side of the chicken breasts and top it with 2 slices of prosciutto. Starting with one of the narrow sides, roll each chicken breast tightly and affix each roulade with 2 - 3 toothpicks. Place the roulades in a rotisserie pan, with the opening facing down. (The roulades can be prepared up to this point, covered and kept in the fridge to be cooked the next day).
2. Apply the melted butter on top of each roulade, and sprinkle the salt and pepper on top. Pour the chicken broth and the white wine in the rotisserie pan. Roast in the oven at 425 F (220 C) for approximately 15 minutes. After 15 minutes, take some of the cooking juices at the bottom of the pan, and apply a bit on top of each roulade. Continue roasting for another 4 minutes, but this time under the gril of the oven, until the top of the chicken is slightly browned.
3. Place the chicken in a serving dish, remove the toothpicks, cover with aluminum foil, and keep warm. Place the rotisserie pan on the stovetop. In a small bowl, mix the flour with the water. While stirring with a whip, pour the flour mix in the rotisserie pan and bring the cooking liquid to a boil. Reduce the heat and let simmer, while stirring, for approximately 4 minutes, or until the sauce has slightly thickened. Add the chopped parsley and mix well.
4. You may or may not want to cut the roulades into slices before serving (I don't bother - it's mainly for presentation that you would slice it first). Serve the chicken with the sauce.