1. Heat the butter in a large pot over medium heat. Sauté the onions for a few minutes until they are soft and translucent. Add the mushrooms and cook for another 5 minutes.
2. Add the dill, paprika, soy sauce, and vegetable broth to the pot. Reduce the heat to medium-low and simmer for 15 minutes.
3. In a separate bowl, whisk together the cornstarch and milk until smooth. Stir this mixture into the soup until it thickens. Cover the pot and simmer for another 10 minutes.
4. Stir in the sour cream, lemon juice, parsley, salt, pepper, and a pinch of cayenne pepper.
5. Cook for another 3 minutes over low heat until the soup is heated through. Do not allow it to boil.
6. Serve the soup in bowls and garnish with sautéed mushrooms, a dollop of sour cream, and parsley. Enjoy with crusty bread on the side.