1 medium celery root, about 12 ounces
1 Granny Smith apple
1/2 cup non-fat plain yogurt
2 tablespoons heavy cream
4 teaspoons Dijon mustard
1 teaspoon coarse salt
1/4 teaspoon freshly ground pepper
Peel celery root; core apple. Grate both on the large holes of a box grater. Toss with lemon juice.
Whisk together yogurt, cream, mustard, salt and pepper. Add to celery root and apple; and toss to combine.
Refrigerate until ready to serve.