Cook pasta in a large pot of boiling water for 8 to 10 minutes. Drain & rinse with cold water. Set aside. While pasta is cooking, heat 2 ts balsamic vinegar & 2 ts water in a large skillet & saute onions & garlic over medium heat. Stir frequently. Add tomato paste, dissolved in 1 cup water. Stir gently & add tomatoes. Cook, stirring occasionally till vegetable are very soft. Dissolve cornstarch in 2 tb cold water & pour into blender. Add miso & cooked vegetables. Puree till smooth. Return puree to skillet. Add 3 tb balsamic vinegar & parsley & heat through. Toss pasta with sauce & serve hot.