Cook the pasta until al dente, allow to cool and place in a salad bowl.
Cut the celery stalk into slices and add. Cut the tomato into cubes, finely dice the onion, finely chop the parsley and also add to the pasta.
Mix the mayonnaise and sour cream in a smaller bowl, add the mustard, vinegar and sugar and season with salt and pepper. Pour over the salad, mix everything together and serve.