Mix the onion with the lemon and orange juices and stock in a bowl to make a marinade. Pour over the chicken or guinea fowl breasts, cover and leave in a cool place for 2 hours. Take the chicken or guinea fowl out of the dish and transfer the marinade to a small pan. Simmer gently, covered, for 20 minutes.
Put the sugar with 1 tablespoon cold water into a small pan over a medium heat, s