Sprinkle the chicken with sage, 1/8 teaspoon salt, and pepper. Place flour in a shallow dish. Dredge chicken in flour. Heat the oil in a nonstick skillet over medium-high heat. Add chicken; cook 4 minutes on each side. Add wine; cover, reduce heat, and simmer for 20 minutes or until a meat thermometer registers 180°.
Place the cornmeal and 1/8 teaspoon salt in a 1-quart casserole. Gradually add water, stirring until blended. Cover. Microwave at high 12 minutes, stirring every 3 minutes. Let stand, covered, 5 minutes.
Remove chicken from pan. Add tomato to pan; cook 1 minute. Stir in juice and prosciutto. Spoon polenta onto plates, and top with chicken. Serve with the sauce. Garnish with sage sprigs, if desired.