• In a stockpot, melt the butter and oil together over medium heat. Add the onion and saute until translucent.
• Add the sliced mushrooms and saute until cooked through and any liquid has evaporated and mushrooms are browned (about 7 to 9 minutes),
• Add the garlic, and cook another minute.
• Stir in flour, mustard powder and paprika and cook another minute.
• Add the sherry, scrape the bottom of the pan to get all the brown bits that may have stuck to the pan. Raise the heat, bring to a boil and cook a further 2 minutes.
• Stir in stock white vinegar, salt and pepper, again scraping up any browned bits on the bottom of the pan. Bring to a boil, lower the heat, and simmer for around 10 minutes.
• Stir in the sour cream and parsley.
• Taste for seasoning and add more salt as needed. Serve hot.

Slow Cooker Method

• Follow the stovetop instructions up to step 3, where the roux is created with paprika, dill, and flour. Gradually whisk in the chicken stock, ensuring a smooth consistency, and transfer the mixture to the slow cooker.
• Add the mushrooms, milk, soy sauce, salt, and pepper to the slow cooker. Stir to combine. Cover and cook on low for 6–7 hours or high for 3–4 hours, allowing the flavors to meld.
• About 30 minutes before serving, stir in the sour cream. Let the soup heat through on low, ensuring it does not boil. Adjust seasoning as needed and serve warm, garnished with fresh parsley or dill, if desired.