For the filling: Sauté the onion and red pepper in the oil until tender. Add the spinach and cook until it wilts. Remove from the heat and allow mixture to cool.
To stuff the chicken: With a sharp knife butterfly the chicken breasts and pound with a flat mallet. Season chicken breasts with salt and pepper to taste. Stuff the prepared chicken with the cooled filling. Roll them up and secure the filling by tying the breast with string.
To cook the breast: In a large heavy bottom sauté pan, heat the olive oil over moderate heat. Sauté the chicken until golden brown. Turn over and continue to cook for 5 minutes. Add wine and stock and let it evaporate by half. Transfer the chicken to an oven proof baking dish. Finish cooking the chicken in a preheated 375° oven until the temperature reaches 165°. About 15 minutes. Serve the chicken sliced with the reduced sauce.