Make the Stuffing: In a bowl, combine the spinach, ricotta, Gorgonzola,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: Slice chicken along side.
Divide the stuffing between the breasts, and insert in flap.
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 serve immediately.