Chill the beef in the freezer for about 15 minutes firm up. Then slice as thinly as you can, put slices between sheets of cling film and bash until nice and thin. Lay out on a platter, cover loosely and refrigerate until required. Blanch the mange tout in boiling water (you will need a rolling boil, not timid bubbles) for one minute and then refresh by running under cold water. This will keep their lovely colour and stop them overcooking. In a bowl, mix the mange tout, mushrooms, spring onions and basil with a spoonful or two of the dressing, and lay it out on a platter or plate. Slice the beef thinly and carefully before layering it up on top of the salad. Drizzle some more dressing on top and serve.