Preheat oven to 350°. Heat a large ovenproof skillet over medium-high heat. Add oil. Add spinach and minced garlic, and sauté for 2 minutes. Combine spinach, blue cheese, flour, and chopped prosciutto. Cut a 1-inch-wide slit into the thick end of chicken breast halves, and carefully cut down to center of chicken to form a pocket. Divide spinach mixture evenly among pockets. Secure with wooden picks. Sprinkle chicken with salt and pepper. Heat pan over medium-high heat; coat pan with cooking spray. Add chicken; sauté 4 minutes. Turn chicken over. Bake at 350° for 12 minutes; let stand 5 minutes.