Drain can of salmon. Make juice up to 300ml / 1/2 pint with water for fish stock. Break fish into large chunks. Heat the oil. Gently fry spring onions and garlic. Stir in flour. Add the fish and vegetable stocks and cider. Mix well. Stir in basil and lemon juice. Bring to the boil. Add prawns, mussels and white fish. Simmer for 10 minutes. Cook mushrooms and onion in salt water. Drain and set aside. Stir salmon into soup. Heat through. Serve garnished with mushrooms and onions. Serves 6. Approx. 290 kcals per serving