Place the dried mushrooms in a heatproof bowl and cover with boiling water, allow to stand while you prepare the rest of the ingredients.
Remove any excess fat from the beef and cut into thin strips, cutting across the grain. Sprinkle with the flour and season with salt and pepper. Toss together so that the meat is coated with the flour set aside.
Slice the onions and mushrooms.
Heat 1 tablespoon of the oil in a large non-stick frying pan and gently cook the onions over a low heat for about 10 minutes until softened and just beginning to colour.
Add the garlic and stir over the heat for a minute or two. Then add the butter and mushrooms. Cook the mushrooms, stirring frequently until softened. Transfer the mushrooms and onions to a plate and set aside.
Heat the remaining oil in the pan and add the beef strips. Toss over a medium-high heat until browned.
Return the cooked mushrooms to the pan. Drain the soaked dry mushrooms reserving 100ml of the soaking water and add to the pan.
Stir the reserved mushroom water into the pan then add the mustard.
Stir in the crème fraîche, season to taste, then cook gently for about 5 minutes. Serve with rice or pasta sprinkled with some snipped chives or parsley
Not suitable for freezing.Nutrition information is calculated using an online nutrition database - is approximate and is meant as a guideline only. (It does not include seasoning with salt and pepper, serving accompaniments or garnish.)