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Heat oil and half the butter in a saucepan.
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Add meat strips in batches, and stir to seal on all sides.
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As each batch cooks, transfer to a strainer sitting over a bowl, and keep juice in bowl.
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When all meat is cooked, add remaining butter to pan.
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Add onion and mushrooms and cook, stirring until onion is translucent and mushrooms are soft.
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Stir in mustard, salt and pepper to taste, and cook briefly.
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Add a new drops of water and scrape up brownings from bottom of pan.
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Pour sour cream into pan and stir to blend.
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Stir in lemon juice and spices.
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Season meat with salt and pepper and stir into sauce.
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Heat through over a gentle flame, stirring. Do not cook meat further or it may toughen.
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Stir in juices drained from meat to finish sauce.
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Serve with pasta or rice.