Place potatoes in a large saucepan. Add water to cover. Cover and bring to a boil; reduce heat and boil gently 10 minutes or until tender. Drain and set aside. Preheat oven to 400 degrees. In a medium saucepan, saute onions in margarine until soft. Blend in flour. Gradually add milk; cook over moderate heat, stirring constantly, until sauce comes to a boil. Remove from heat and stir in the parsley, dill weed and pepper. Gently mix in the cheese and finally the potatoes. Turn into a 2 quart baking dish. Bake, uncovered, about 25 minutes. Makes 8 servings.