Heat oil in a large frying pan, then brown the chicken breasts for 2 mins on each side. Remove from the pan and keep warm.
Add the balsamic vinegar, sugar and tomato puree to the pan, then bring to a simmer for 15 mins,
Put the chicken breasts back in the pan, and then simmer for 15 mins, turning the chicken occasionally.
Meanwhile cook the potatoes in a pan of boiling water until tender, then mash with butter, yogurt and spring onion.
Serve the chicken in slices on a bed of mash, drizzled with the balsamic sauce.