Preheat the oven to 425 degrees F (220 degrees C).
Slice about 1/4 inch off of the tops and bottoms of the onions. Cut in half crosswise leaving the skin on, and arrange cut side up in a shallow baking dish. Pour the chicken stock over and around the onion halves, then drizzle with olive oil. Season liberally with salt, pepper and rosemary. Place on top of a cookie sheet to catch any splatters that may occur.
Bake uncovered for 1 hour in the preheated oven, or until onion is easily pierced with a knife and liquid has reduced by 1/2. Remove the dish from the oven, and pour the cream over the onions. Return to the oven, and continue to bake uncovered until the pan juices have thickened slightly and onions are browned, up to 30 minutes.