Melt butter in a soup pot over low heat while you chop mushrooms into small pieces. Add the mushroom to the butter and cover the pot with a lid. Cook the mushrooms down until they’re dark and tender. Season the mushrooms with onion powder, paprika, and fresh dill.
Let the spices cook with the mushrooms for about 30 seconds before adding Worcestershire sauce. Stir the mushrooms and let them cook longer – for another 30-60 seconds. Then, pour the vegetable broth into the pot. Bring the pot to a simmer.
While the soup is coming to a simmer, pour heavy cream into a separate dish. Whisk tapioca flour into the cream, then pour the thickened mixture into the soup pot. Stir the soup together and simmer until it reduces and thickens slightly.
Pour lemon juice into the soup and stir. Taste the soup and salt and pepper it to your taste. Finally, with the soup at a low heat (no longer simmering), whisk in sour cream. Whisk the sour cream until it blends into the soup. When the soup is done, you can garnish it with more chopped dill, sour cream, or parsley.