Melt butter in a skillet over medium-low heat.
Add mushrooms and onions and sauté until tender, about 8 minutes.
Season to taste with salt and pepper.
Add garlic and cook for 2 minutes.
Add flour and cook for an additional 2 minutes.
Quickly whisk in cream and chicken broth until smooth.
Bring to a boil and boil for 1 minute.
Remove from heat and taste, adjusting seasoning if needed.
Allow to cool slightly before transferring to a jar or freezer-safe container.
Once soup is completely cool, store it in the refrigerator or freezer.
To reconstitute, add 1 ½ cups of liquid, such as chicken broth, milk, water, or a combination.