For the dressing
Mix together sour cream, sugar, prepared horseradish, dry mustard, and salt in a small bowl.
Gradually stir in the herb-flavored vinegar using a wire whisk.
Cover and chill.
Place the torn spinach in a large bowl.
Spoon the cottage cheese on top of the spinach.
If desired, sprinkle with the walnuts.
Pour dressing over salad.
Toss lightly to coat well and serve.