Flatten breasts using a meat pounder or mallet. When you are done, they should be about ¼ inch thick.
Melt the butter in a large skillet. Raise the heat, add the chicken and cook for 3 minutes per side, or until they are lightly colored. Remove from the skillet and reserve.
Add the onion to the fat in the pan and cook, covered, over low heat until tender, about 15 minutes.
Add the vinegar, raise the heat and cook, uncovered stirring occasionally, until vinegar is reduced to a syrupy couple of spoonfuls. Whisk in the chicken stock, heavy cream, and crushed tomatoes and simmer for 2 minutes. (You can omit the tomatoes – one tablespoon out of a can is very little – but if you use the tomatoes, the dish will taste brighter and more vibrant.)
Return the chicken to the pan and simmer them gently in the sauce, basting often, until they are just done and the sauce has been reduced or thickened slightly, about 5 minutes. Be careful to not overcook the chicken.
Remove the chicken with a slotted spoon and arrange on a serving platter. Add the raspberries (if using) 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.
Pour sauce over chicken and serve immediately.
Goes great with a fresh green vegetable and garlic mashed potatoes!