In a large saucepan, bring the chicken stock to a boil. Stir in the tom yum paste and garlic, and cook for about 2 minutes. Stir in the lemon grass and kaffir lime leaves. Place the chicken in the saucepan, and cook 5 minutes, or until no longer pink and juices run clear.
Mix in the mushrooms. Add the fish sauce, lime juice, and green chile pepper. Continue cooking until well blended. Remove from heat, and serve warm with the coriander and basil.
Prawns (approximately 1/2 pound) may be substituted for chicken in this recipe. Cook them only until opaque. If kaffir lime leaves are not available, substitute 1 tablespoon lime zest.