Boil potatoes in water just until tender. Drain and place in ice water; drain well. Quarter potatoes; chill.
Sprinkle fish evenly with 1/4 teaspoon of salt and pepper; place on a broiler pan coated with cooking spray. Broil 5 minutes on each side or until fish flakes easily when tested with a fork. Let stand 10 minutes; chill.
Combine sour cream and next 7 ingredients in a small bowl. Cover and chill.
Place 1 fillet on each of 4 plates; add 8 potato quarters, and sprinkle with 1/4 cup green peas. Drizzle one-fourth of dressing over each; garnish with a lemon wedge and dill sprig, if desired.