Start by heating the butter and olive oil in a large pot over medium heat. Once melted and warm, add the chopped onion. Cook for about 5–6 minutes, stirring often, until the onion becomes soft and translucent. This step builds the base flavor, so don’t rush it.
Add the sliced mushrooms and cook for 8–10 minutes. At first, they’ll release moisture, then slowly begin to brown. Let them cook until most of the liquid evaporates and the mushrooms deepen in color.
Sprinkle the flour over the mushrooms and stir well. Cook for about 1 minute to remove the raw flour taste.
Lower the heat slightly and stir in the paprika, thyme, salt, and pepper. Paprika burns easily, so keep stirring and don’t let it sit directly on high heat.
Slowly pour in the broth, stirring constantly to prevent lumps. Bring the soup to a gentle simmer. Add the milk and soy sauce, then simmer for 10–15 minutes, stirring occasionally.
Reduce the heat to low. In a small bowl, whisk the sour cream with a few spoonfuls of hot soup to temper it. Slowly stir it back into the pot. Add the lemon juice, taste, and adjust seasoning.
Serve warm, topped with fresh parsley.