In a 5-quart Dutch oven or heavy-bottomed pot, melt the butter over medium heat. Add the onions and sauté for 3 to 4 minutes until they are softened and translucent.

Add the mushrooms to the pot and cook for an additional 5 minutes.

Stir in the dill, paprika, soy sauce, and vegetable broth. Lower the heat to medium-low and let the mixture gently simmer, covered, for 15 minutes.

In a small bowl, whisk together the cornstarch and milk until fully blended. Pour this mixture into the soup and stir until well incorporated.

Replace the cover on the pot and let the soup gently simmer for another 10 minutes, stirring occasionally.

Add the sour cream, lemon juice, parsley, salt, pepper, and a pinch of cayenne to the pot. Stir to combine.

Continue to cook the soup over low heat for about 3 minutes, ensuring it is warmed through but not boiling.

Serve the soup in bowls, garnishing each with sautéed mushrooms, a dollop of sour cream, and additional parsley.

Enjoy the soup with some crusty bread on the side.