Place vinegar in 2-quart saucepan. Core, seed and slice quinces and place in pan with vinegar and add sugar. Reduce by two thirds. Place in food processor and blend until smooth and thick. Set aside.
Bring 2 quarts water to boil and blanch beans 1 to 2 minutes or until crisp and bright green. Shock in ice water. Drain and toss with watercress, hazelnut oil, hazelnuts and 2 tablespoons quince vinegar. Season with salt and arrange on large platter.
Place foie on hot grill and cook to medium-rare, about 3 to 4 minutes per side. Remove, slice and arrange on platter. Drizzle with quince vinegar and serve immediately.