Make the Stuffing: In a bowl, combine the spinach, ricotta, Gorgonzola, and bacon and mix well. Season with salt and pepper. Add the egg, mix well, and set aside.
Make the Sauce: Combine the wine and stock in a non-reactive saucepan. Bring to a boil and reduce to a sauce-like consistency. Whisk in the mustard, lemon juice, and season with salt and pepper, to taste. Set aside.
Make the Chicken: Place each chicken breast between 2 doubled-up sheets of plastic wrap, and pound to an even 1/4-inch thickness with a meat-pounder or rolling pin. Divide the stuffing between the breasts, mounding it along the center of each. Fold the bottom edge of each breast over the stuffing, fold in the sides, and roll forward until completely wrapped, to form a tight rolled package. Secure each flap with a toothpick. Season the chicken all over with salt and pepper, to taste.
Heat the oil in a large skillet over medium heat. Add the chicken and cook, turning occasionally, until browned, about 2 minutes per side. Cover the skillet, turn the heat down to low, and cook until just cooked through, about 5 minutes more.
Transfer the chicken to a cutting board and let rest for 5 minutes.
Meanwhile, pour the sauce into the skillet over high heat and cook, stirring and scraping the bottom with a wooden spoon.
Slice the chicken into medallions, divide among plates, and spoon some of the sauce over each. Garnish with parsley and erve immediately.