My kids love mushroom soup too and I have tried both ways (through trial and error last time) suggested by 2 mummies - add stock with left hand & stir with right hand, and add stock bit by bit. Though leh chey, but I totally agree with DIYmummy - got to add bit by bit else got lumps here and there - not nice. Here's the recipe that I found in the internet which we like best 
Tip 1: I do the 'Chinese style' ;-p I fry onion until fragrant, then add the mushroom.
Tip 2: I blend the other half of mushroom. I find that the finer you blend, the better it taste.
Tip 3: Amount of flour - I go by agaration - feel the stickiness of the paste when stirring. Hard to explain...
Tip 4: If I cook a big pot for a small party, I use 1 cup of heavy cream. If I cook for just my family, I use about 1/2 to 3/4 of the normal cream (from supermarket).
Cream of Mushroom:
INGREDIENTS:
4 Tablespoons butter, divided use
8 ounces mushrooms
1 teaspoon salt
1 medium sweet onion, chopped
1/4 cup flour
1/4 teaspoon white pepper, or to taste
2 cans (10-3/4 ounces each) chicken broth
1/8 tsp ground nutmeg
1 cup heavy cream
Chopped parsley for garnish
PREPARATION:
Clean mushrooms with a mushroom brush or damp paper towel. Cut half of the mushrooms into slices. Chop the remainder.
Melt half of the butter in a large saucepan and saute sliced mushrooms until golden. Remove and reserve sliced mushrooms. Add remaining butter to the same pan, along with the chopped mushrooms and sweet onions. Sprinkle with salt. Cook, stirring often, until onion is soft.
Return sliced mushrooms to the pan. Add flour and cook, stirring constantly about 2 minutes, until smooth. Slowly add chicken broth while stirring. Simmer, stirring often, until thickened. Add nutmeg and pepper. Taste and adjust seasoning, if need be.
Add heavy cream and bring to a simmer. (Do not boil.) Ladle into soup bowls and garnish with chopped parsley.
Tip 1: I do the 'Chinese style' ;-p I fry onion until fragrant, then add the mushroom.
Tip 2: I blend the other half of mushroom. I find that the finer you blend, the better it taste.
Tip 3: Amount of flour - I go by agaration - feel the stickiness of the paste when stirring. Hard to explain...
Tip 4: If I cook a big pot for a small party, I use 1 cup of heavy cream. If I cook for just my family, I use about 1/2 to 3/4 of the normal cream (from supermarket).
Cream of Mushroom:
INGREDIENTS:
4 Tablespoons butter, divided use
8 ounces mushrooms
1 teaspoon salt
1 medium sweet onion, chopped
1/4 cup flour
1/4 teaspoon white pepper, or to taste
2 cans (10-3/4 ounces each) chicken broth
1/8 tsp ground nutmeg
1 cup heavy cream
Chopped parsley for garnish
PREPARATION:
Clean mushrooms with a mushroom brush or damp paper towel. Cut half of the mushrooms into slices. Chop the remainder.
Melt half of the butter in a large saucepan and saute sliced mushrooms until golden. Remove and reserve sliced mushrooms. Add remaining butter to the same pan, along with the chopped mushrooms and sweet onions. Sprinkle with salt. Cook, stirring often, until onion is soft.
Return sliced mushrooms to the pan. Add flour and cook, stirring constantly about 2 minutes, until smooth. Slowly add chicken broth while stirring. Simmer, stirring often, until thickened. Add nutmeg and pepper. Taste and adjust seasoning, if need be.
Add heavy cream and bring to a simmer. (Do not boil.) Ladle into soup bowls and garnish with chopped parsley.