Separat breasts if they are attached and cut each in half crosswise. Trim any fat from chicken. Melt butter in a large flame-proof casserole. Cook onion over moderate heat ntil soft, about 3 minutes. Add chicken and cook until no longer pink on the outside, 5 to 7 minutes. In a small bowl, whisk together the mustards and wine. Pour over chicken and bring to a boil. Add broth. Simmer partially covered for 25 to 30 minutes, until chicken is tender and cooked through, Remove from heat and stir in sour cream. Nutritional info per serving: 555 cal; 67g pro, 7g carb, 24g fat (41%)