1. Combine first 8 ingredients in a large bowl, stirring with a whisk until smooth. Add fish; toss to coat. Cover and chill 2 hours.
2. Remove fish from marinade; discard marinade and solids. Combine fish and peanut oil in a bowl; toss to coat.
3. Heat a large heavy skillet over high heat. Coat pan with cooking spray. Add onion, 1/4 cup dill, and green onions; stir-fry 3 minutes. Add soy sauce and sugar; stir-fry 1 minute. Arrange onion mixture on a platter. Return pan to high heat. Add half of fish to pan; cook 3 minutes. Turn fish over; cook 1 minute or until fish flakes easily when tested with a fork. Place fish on platter over onion mixture. Repeat procedure with remaining fish. Sprinkle with remaining 1/4 cup dill and peanuts. Serve with lemon wedges.