Prepare your beef. Pat the beef strips dry with kitchen paper — moisture is the enemy of browning. Season them generously with a little salt and pepper. Heat 1 tablespoon of the oil in a large frying pan over a high heat until it shimmers and begins to smoke gently. Add the beef in a single layer (you may need to do this in two batches) and sear without moving for 60–90 seconds until the underside is a deep, dark brown. Flip and cook for another 30 seconds. The beef should be browned on the outside but still pink inside. Transfer to a plate and set aside.

Brown the mushrooms. Reduce the heat to medium-high and add the butter and remaining oil. Tip in the sliced mushrooms and spread them into an even layer. Let them cook undisturbed for 2–3 minutes until they release their moisture and develop a golden-brown colour. Stir once, then continue cooking for another 3–4 minutes, until the mushrooms are deeply bronzed and smell wonderfully nutty and earthy.

Soften the onion and garlic. Add the sliced onion to the pan with the mushrooms and cook for 4–5 minutes, stirring often, until the onion has softened and turned translucent with pale gold edges. Stir in the chopped garlic and cook for a further 30 seconds until fragrant — you’ll notice the aroma become sweeter and more pungent.

Make the roux. Sprinkle the flour over the vegetable mixture and stir constantly for 1 minute. The flour will coat the mushrooms and onions and turn a very light biscuit colour. This step removes the raw flour taste and helps thicken the sauce later.

Build the sauce. Gradually pour in the beef stock, stirring continuously to prevent lumps from forming. Bring the liquid to a gentle simmer — you’ll see small bubbles breaking across the surface. Let it bubble gently for 3–4 minutes, stirring occasionally, until the sauce has thickened enough to coat the back of a spoon.

Finish the sauce. Stir in the Dijon mustard until fully combined, then remove the pan from the heat. Allow the sauce to cool for 1 minute before stirring in the soured cream or crème fraîche — this prevents it from splitting. Return the pan to a very low heat and stir until the sauce is warm, smooth and a pale, creamy beige colour. Taste and adjust the seasoning with salt and pepper.

Return the beef and serve. Add the reserved beef (along with any juices on the plate) back into the pan. Stir gently and warm through for 1–2 minutes over low heat — no longer, or the beef will overcook and toughen. The sauce should be rich, glossy and flecked with dark mushrooms. Sprinkle with the chopped parsley and serve immediately over cooked egg noodles, pappardelle or steamed rice.