2 whole boneless, skinless chicken breasts, about 2 lbs.
2 tbsp. sweet butter
1/4 c. finely chopped yellow onion
4 tbsp. raspberry vinegar
1/4 c. chicken stock or canned chicken broth
1/2 c. heavy cream or creme fraiche
1 tbsp. canned crushed tomatoes
16 fresh raspberries (optional)

1. Cut each chicken breast into halves along the breastbone line. Remove the filet mignon, the finger-size muscle on the back of each half, and reserve for another use. Flatten each breast half by pressing it gently with the palm of your hand.

2. Melt the butter in a large skillet. Raise the heat, add the breasts and cook for about 3 minutes per side, or until they are lightly colored. Remove from the skillet and reserve.

3. Add the onion to the fat in the pan and cook, covered, over low heat until tender, about 15 minutes.

4. Add the vinegar, raise the heat and cook, uncovered, stirring occasionally, until vinegar is reduced to a syrupy spoonful. Whisk in the chicken stock, heavy cream or creme fraiche and crushed tomatoes and simmer for 1 minute.

5. Return the breasts to the skillet and simmer them gently in the sauce, basting often, until they are just done and the sauce has been reduced and thickened slightly, about 5 minutes; do not overcook.

6. Remove breasts with a slotted spoon and arrange on a heated platter. Add the raspberries to the sauce in the skillet and cook over low heat for 1 minute. Do not stir the berries with a spoon; merely swirl them in the sauce by shaking the skillet.

7. Pour sauce over breasts and serve immediately.