Remove mushroom stems level with caps, then finely chop stems and thinly slice caps.
In a large saucepan over medium heat, melt butter. Add mushrooms and onions and sauté 5 minutes. Add flour and stir well.
Add stock, salt and pepper and bring to boil, stirring constantly. Add rice and bay leaf. Reduce heat and simmer, covered, 15 to 20 minutes or until rice is tender. Remove bay leaf. Sprinkle parsley over each serving of mushroom soup with onion.