In a large skillet, saute onion in butter until tender. Add water and dill; bring to a boil. Reduce heat; simmer, uncovered, for 4-5 minutes. Add milk; stir in lemons and pepper. Top with fillets. Cover and simmer for 12-15 minutes or until fish flakes easily with a fork.
Transfer fish to a serving platter and keep warm. Strain cooking liquid, reserving lemons, onion and dill; serve with fish. Yield: 8 servings.