My dad is a mushroom nut. He LOVES his mushrooms. Where he and my mom are from in Poland, there are lots of forests where mushrooms grow wild, and they are a staple of cooking in the region.
My dad uses them fresh, dried, dehydrated, whatever. Mushroom soup (Zupa Grzybowa) was on our table a lot. This is simple to make and really so full of flavor. My mother makes the best. This is based on her recipes (which is not written down, so I've translated her "little of this, little of that" method!)
You can also add a hearty noodle to this soup, or vegetables like carrots. Also, now they even sell "Zupa Grzybowa" flavor packets in the store; I have used these sometimes to enhance the flavor of the soup, but not as a substitute for using real mushrooms or real stock.
Put dried mushrooms in a pot and cover them with boiling water. Cover pot and let stand to soak until they are soft. This depends on the mushroom you use. This may take a few minutes, or maybe sometimes an hour.
After they are softened, rinse them and chop them up into diced bits, or into strips.
In your soup pot, add all mushtooms and the broth. Bring to a boil and then reduce to simmer.
In a saucepan, sautee onions in all the butter until brown and tender. Add onions and butter to the pot.
In a small bowl, mix cornstarch/flour with a ladle or two of the hot soup. Whisk until all lumps are gone. Add it all back to the soup, or strain through small strainer if you still think you have lumps left.
Stir in the sour cream. Salt and pepper to taste. Also add vegeta if you have it / if you want to. At this point, you may want more sour cream so add 8 oz instead of 4. I don't like lots of sour cream so I just do 4.
Garnish with chopped dill / parsley / pepper when serving.