Chicken Dijon

2 tbsp butter
4 boneless skinless chicken breasts
salt and pepper
1 large onion, chopped
1 tbsp all purpose flour
1 cup chicken stock
1 tbsp dijon or grainy mustard

In a heavy skillet, melt butter over medium high heat; cook until chicken is browned, about 3 minutes on each side, adding more butter if necessary. Sprinkle with salt and pepper to taste. Add onion and cook, stirring until tender. Remove chicken from pan and keep warm. Sprinkle flour over onion and cook, stirring for 1 minute. Stir in stock and brint to a boil, stirring up brown bits from bottom of pan. Return chicken to pan; reduce heat and simmer for 3-5 minutes or until chicken is no longer pink inside. Remove from heat and stir in mustard.