Duck breast with orange sauce

2 breasts 2 oranges, 1cuil. tablespoon of sugar, 1 tsp. tablespoons of wine vinegar, 20cl chicken stock, 2 tbsp. tablespoon Grand-Marnier, 1 tsp. coffee of starch, salt, pepper .
Wash the oranges, remove the peel and cut them into fine julienne. Scald them for 2 minutes, refresh and drain them. Peel the oranges and remove the quarters.
Put the sugar and vinegar in a saucepan and cook over low heat to obtain a dark blond caramel. Add the chicken stock, orange juice and zest. Add salt and pepper and cook for 20 minutes over low heat.
Score the skin side of the duck breast with a knife and season with salt and pepper. Heat a casserole or large frying pan. Cook the duck breasts on the skin side for 2 minutes on high heat and then 3 minutes on low heat. Turn them over and cook for 3 minutes on the flesh side (pink), then place them under a sheet of aluminum foil.
Mix the Grand Marnier and the cornstarch in a bowl. Stir this mixture into the sauce. Add the orange quarters and let it simmer for 5 minutes.
Slice the duck breasts, arrange them on a plate and lightly coat them with the sauce. Serve immediately with the remaining sauce on the side.