1. Combine sour cream, 2 tablespoons parsley, mustard, and 1/8 teaspoon pepper, stirring with a whisk until blended. Set aside.
2. Heat a large nonstick skillet over medium heat. Add 1 teaspoon canola oil to pan, and swirl to coat. Add onion and celery; cook 5 minutes or until tender, stirring occasionally. Remove from heat; cool slightly.
3. Combine shrimp and crab in a large bowl. Stir in onion mixture, remaining pepper, remaining 1/4 cup parsley, cheese, salt, egg whites, and egg; stir gently. Add panko; stir gently. Divide mixture into 8 equal portions; flatten to 1/2-inch-thick patties.
4. Heat skillet over medium-high heat. Add remaining 2 tablespoons oil to pan; swirl to coat. Add patties to pan; cook 3 minutes or until lightly browned. Carefully turn over; cook 3 minutes or until done. Serve with sauce.