Place the mushrooms, onions, mustard and stock in a saucepan and simmer gently for 20 minutes. Save a few mushroom slices for decoration later.
Add the remaining ingredients, except for the lemon juice. Heat through, but do not boil.
Garnish with a few slices of mushroom (saved from before) and a squeeze of lemon juice. Serve.
NOTES: Mushroom soup with whisky -- This is my family's favourite mushroom soup recipe. The recipe originally came from a small booklet produced by Carnation.