Place mustard seeds and celery seeds in a mortar. Using a pestle, grind until coarse; set aside.
Combine cabbage and carrot in a large bowl; set aside.
Combine seed mixture, red bell pepper, and next 6 ingredients in a food processor; process until smooth. With processor running, slowly pour oil through chute. Add sour cream; pulse until smooth.
Pour mixture over cabbage and carrot, tossing well. Cover and chill 1 hour.