Combine onion, bouillon, water and lemon juice in skillet large enough to hold fish in single layer. Bring to a boil. Add salmon and simmer tightly covered about 10 minutes, or until fish flakes easily with a fork.
In small saucepan, melt butter over medium heat and saute chopped onion until tender, about 3 minutes. Stir in flour, dill weed, pepper. Add milk, mixing well. Heat until mixture boils and thickens, about 2 minutes, stirring constantly. Pour sauce over salmon and serve.