Sprinkle one side of chicken with salt and pepper. Heat a large skillet over medium/high heat until hot. Add half of butter and swirl to coat pan. Add chicken breasts, seasoned side down and season remaining side. Brown, turning once, until well browned on both sides. Transfer to a plate and cover with foil to keep warm.
Melt remaining butter in skillet. Cook potatoes, onions and rosemary, stirring for 2 minutes or until some liquid is released. Cover, reduce heat to medium and cook, shaking pan often for about 15 minutes or until potatoes and onions are almost tender and starting to brown. Increase heat to medium/high and add vinegar. Boil, scraping up and bits stuck to pan, until reduced and syrupy. Pour in stock and bring to a boil. Return chicken to pan with any accululated juices, spooning some of the sauce over chicken. Cover, reduce heat and simmer for about 10 minutes, turning chicken halfway until potatoes are tender and chicken is no longer pink inside.
Season with salt and pepper to taste and garnish with fresh rosemary