Preheat the oven to 170 degrees Celsius. Mix the coriander, cumin, ginger and salt together. Score the skins of the duck breasts and rub each piece all over with the spice mixture. Fry the pieces in a hot pan skin-side down, for three minutes. Place the pieces in a non-oiled pan (skin side up) and roast in the oven for eight minutes. Remove and let them rest for 10 minutes in a warm place. 

For the sauce, melt the butter in a saucepan and add sugar. Stir in chicken stock, soy sauce, vinegar and grape juice. Simmer over low heat until the liquid has reduced by one-third. Thicken by mixing the starch with cold water and then stirring the mixture into the sauce. Add the grapes and bring to a boil. Serve with the duck breast and mashed potatoes.