1. Melt the butter in a large skillet over medium heat.
2. Flatten each chicken breast slightly by pressing with the palm of your hand.
3. Add chicken breasts to the skillet and increase heat to medium-high. Cook until breasts are lightly colored on each side - about 3 minutes per side. Remove the chicken from the skillet and set aside.
4. Add the onion to the skillet and reduce heat to medium-low. Cook until tender - about 3 minutes.
5. Add the vinegar to the onions and increase heat to medium-high. Cook, stirring occasionally, until the mixture is slightly thickened and syrupy - about 5 minutes.
6. Whisk in the broth, cream, and tomato sauce. Simmer over medium-low heat for 1 minute.
7. Return chicken breasts to the skillet and simmer until they are just done, spooning the sauce over them as they simmer - 5-10 minutes, depending on the thickness of the breasts.
8. Remove the chicken breasts with a slotted spoon and place on a serving platter.
9. Add the raspberries to the sauce in the skillet and poach over low heat for 1 minute. To avoid crumbling the berries, swirl the skillet slightly instead of stirring. (Optional)
10. Pour the sauce and raspberries over the chicken breasts and serve immediately.
2 boneless and skinless chicken breast.
2 tablespoons butter.
1/4 cup onion, finely chopped.
4 tablespoons raspberry vinegar.
1/4 cup chicken broth.
1/4 cup heavy cream.
1 tablespoon tomato sauce.
16 fresh raspberries.