Heat the unsalted butter in a large soup pot or Dutch oven over medium heat

Add the finely chopped onion and cook until soft and translucent, about 5 minutes

Stir in the minced garlic and sauté until fragrant, about 1 minute

Add the sliced mushrooms and cook until they’ve released their liquid and shrunk in size, about 8–10 minutes

Sprinkle the Hungarian sweet paprika and smoked paprika (if using) over the mushrooms and stir to coat

Pour in the vegetable or chicken broth, then add the soy sauce and bring the mixture to a gentle simmer

Whisk the all-purpose flour into the whole milk in a small bowl until smooth, then gradually stir this into the simmering soup to help thicken it

Cook the soup over medium-low heat for about 5 minutes until it begins to thicken slightly

Reduce the heat to low and whisk in the sour cream until fully incorporated, taking care not to boil (which can cause curdling)

Season with salt and freshly ground black pepper to taste

Ladle the hot soup into bowls and garnish with chopped fresh parsley before serving