Set a large 6 quart saucepot over medium heat. Add the butter, onions, and celery. Sauté for 3-5 minutes to soften. Then move the veggies to the side of the pot and add in the mushrooms, 1 teaspoon salt, and ½ teaspoon pepper. Sauté another 8-10 minutes, stirring regularly.
Stir in the soy sauce, dill, smoked paprika, lemon juice, and vegetable broth. Simmer for 10-15 minutes.
Meanwhile, set out a medium bowl. Add the sour cream and flour. Stir until smooth.
Ladle some of the soup broth into the sour cream mixture, stirring constantly so the sour cream doesn’t curdle. Once the mixture is thin, whisk the sour cream mixture into the soup base, whisking continually. (This method ensures there are not curdles cream clumps in the soup.)
Simmer another 3-5 minutes to thicken. Taste, then add additional salt and lemon juice if needed.
Serve warm with a sprinkling of fresh herbs and a dollop of sour cream on top.
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Notes
Allow the soup to cool completely before transferring leftovers to a container with a lid, and keep in the fridge for up to 4 days.Because of the sour cream used in this recipe - both dairy and vegan versions - I do not recommend that you freeze this cream of mushroom soup.