Melt butter in a large saucepan over med heat and cook the chopped onions and celery until soft, about 6 minutes. Add the garlic and cook, stirring, for 1 minute. Add the mushrooms and cook 4 to 6 minutes, stirring occassionally, until the mushrooms are soft and have released their liquid.
Sprinkle the flour over the mushrooms, salt, and pepper, and stir to combine. Cook for 2 minutes over medium-low heat, stirring constantly until the liquid is absorbed by the flour and vegetables are coated.
Add half the chicken stock in a steady stream while stirring to begin to create a sauce. Once the first half is incorporated, add the remaining stock.
Stir in the cream in the same way and simmer just until the soup begins to thicken, 8-10 minutes. Taste and season with sea salt and pepper, as desired.
Save a step and buy cremini mushrooms already sliced.
Add broth or milk to thin when reheating, if needed.
To thicken the mushroom soup without adding flour, transfer 1/3 of the sauteed onions, celery, and mushrooms to a food processor or blender. While the machine is running, pour in one cup of chicken stock in a steady stream until solids are pureed and form a thick paste. Incorporate this paste back into the pot with enough remaining chicken broth to make a loose sauce. Continue with the recipe as written.
1 (10.5 ounce) can of concentrated cream of mushroom soup makes 21 ounces of soup (a little more than 2 1/2 cups).
Dried mushroom TIPS:
Dried mushrooms must be re-hydrated before use
Water, broth, or wine can be used to re-hydrate dried mushrooms
The liquid used to re-hydrate can and should be used in the recipe, but strain it and taste first; discard if overly bitter