Gently pound chicken to 1/4 inch thickness; sprinkle with salt. In medium bowl, mix together mint, basil, thyme and tomatoes; sprinkle over chicken. Roll up each chicken breast and fasten with toothpicks. In large frypan, place olive oil and heat to medium temperature. Add chicken and cook, turning, about 10 minutes or until brown on all sides. Stir in chicken broth and lemon juice, scraping any brown bits from bottom of pan. Add onion, cover and simmer about 10 minutes or juices run clear and fork can be inserted in chicken with ease. Remove chicken rolls from frypan, set aside and keep warm. In small dish, mix cornstarch and vermouth until smooth. Add to frypan and cook, stirring, until mixture is clear; stir in capers. To serve, place half the sauce in serving dish, slice chicken and place, overlapping slices, on top of sauce. Spoon remaining sauce over chicken and garnish with lemon slices and mint sprigs.