In a 3-quart saucepan cook mushrooms, celery, and green onion in hot margarine or butter until vegetables are tender, stirring often.
Stir in flour and salt. Add milk and half-and-half or light cream all at once. Cook and stir over medium heat until mixture is thick and bubbly. Stir in potatoes, crabmeat, and dill or dillweed. Heat through, stirring occasionally.
Ladle into soup bowls. Garnish each serving with fresh dill, if desired. Makes 6 side-dish servings.