Here’s one of those old classics that is somewhat out of vogue with the modern style, being a rich dairy sauce. Nonetheless, a good one takes a bit of beating and it is quick and easy to make. So, I think it deserves to be kept in focus. I like to serve this with flat ribbon noodles and a steamed green vegetable such as broccoli.
Melt half the butter in a large sauté pan and gently cook the onions for about 5 minutes, or until they start to colour a little. Add the mushrooms and cook for 3 more minutes, stirring frequently to cook them evenly. Remove the vegetable mixture to a plate and set aside.
In the same pan, melt the remaining butter over higher heat and add the beef strips. Sauté these for 4 minutes stirring constantly to brown them evenly. Return the onion and mushroom mixture to the pan, season with salt and pepper, and stir to mix with the beef. Cook for a further minute.
Mix the mustard together with the sour cream and stir this into the beef and vegetables. When all is well blended, cook over high heat for a minute but do not allow the sauce to boil. Remove from the heat and adjust the seasoning to taste.