Daria's Fresh Mushroom Soup

2 cups 1/2 & 1/2
1 medium onion
1/2 stick unsalted butter
3/4 lb thinly sliced mushrooms
4 tsp flour
1 cup beef broth
1/2 bay leaf
1/4 tsp salt
1/8 tsp black pepper
1 tsp lemon juice

Bring 1/2 & 1/2 to a boil in a heavy sauce pan. Remove from heat and keep warm. Cut 3/4" crosswise slice of onion and reserve the rest for later. Heat butter, whisk in 1 tsp flour. Add mushrooms and cook for 8 - 10 minutes, until brown. Add onion slice, sprinkle with remaining flour and cook, stirring for two minutes. Add broth slowly, stirring constantly. Add 1/2 & 1/2, bay leaf, salt and pepper. Reduce heat to low and cook or 10 minutes, stirring occasionally. Discard onion and bay leaf. Stir in lemon juice and serve.