1. Heat a large nonstick skillet over medium-high heat. Coat pan with cooking spray. Add prosciutto to pan; sauté 2 minutes or until browned. Add rosemary and garlic to pan; sauté 1 minute. Spoon prosciutto mixture into a bowl; cool to room temperature. Stir in fontina cheese; set aside.
2. Cut a horizontal slit through thickest portion of each chicken breast half to form a pocket. Stuff about 2 tablespoons prosciutto mixture into each pocket; press lightly to flatten. Sprinkle chicken evenly with pepper.
3. Place crackers in a food processor; process 2 minutes or until finely ground. Place cracker crumbs in a shallow dish. Place flour in another shallow dish. Combine egg whites and mustard in another shallow dish, stirring mixture with a whisk.
4. Working with one chicken breast half at a time, dredge chicken in flour, shaking off excess. Dip chicken into egg white mixture, allowing excess to drip off. Coat chicken completely with cracker crumbs. Set aside. Repeat procedure with remaining chicken, flour, egg white mixture, and cracker crumbs.
5. Heat pan over medium-high heat. Add oil to pan, swirling to coat. Add chicken to pan; reduce heat to medium, and cook 10 minutes on each side or until browned and done.
Finely ground saltine cracker crumbs create a golden crust. You can save the step of making your own crumbs if your supermarket stocks cracker meal; if so, start with about 1 1/2 cups.