Place each chicken breast half between 2 sheets of heavy-duty plastic wrap; flatten to 1/4-inch thickness using a meat mallet or rolling pin. Combine flour and oregano in a shallow dish; dredge chicken in flour mixture.
Heat oil in a large nonstick skillet over medium-high heat. Add chicken; cook 5 minutes on each side. Remove chicken from pan; keep warm. Add onion and garlic to pan; sauté 2 minutes. Stir in broth, raisins, and lemon juice; cook 3 minutes, scraping pan to loosen browned bits. Return chicken to pan. Cover, reduce heat, and simmer 10 minutes or until chicken is done. Remove chicken from pan; keep warm. Add capers and cheese to pan, stirring with a whisk; top each chicken breast with 1/4 cup sauce and 1 lemon slice.