In a large skillet, combine the first five ingredients; bring to a boil. Add salmon. Reduce heat; cover and simmer for 15-20 minutes or until fish flakes easily with a fork.
Meanwhile, in a saucepan, saute onion in butter until tender. Stir in flour, dill, salt and pepper until blended; gradually add milk. Bring to a boil; cook and stir for 2 minutes or until thickened. Serve with the salmon.