Preheat a Dutch oven or another soup pot over medium heat.
Melt butter in the pot and add onions. Cook onions until golden brown.
Add mushrooms and cook, stirring occasionally, for about 10 minutes.
Sprinkle flour over mushrooms and onions and stir well.
Pour in stock while stirring and bring the mixture to boil. Once the soup starts to boil, turn it down to medium-low.
Add bay leaf, minced sage, garlic powder, salt, and pepper to taste. Stir and cook uncovered for about an hour. Stir the mixture occasionally.
Take out the bay leaf.
Stir in heavy cream and cook for a couple more minutes.
**Blended option: you can also make this soup smooth instead of chunky by blending the mixture before adding heavy cream. Just transfer the entire content of the pot into a blender (remember to take out the bay leaf first) and pulse until all smooth. Pour the mixture back into the pot and stir in heavy cream. Cook for a couple more minutes.**
Store mushroom soup in an air-tight food storage container in the refrigerator for 3-4 days.
Make it thicker:
It's very easy to make the soup thicker if you wish and all you have to do is increase the amount of flour. This soup is pretty brothy as is so if you want it thicker, use double the amount of flour, which would be 6 tablespoons.