250g/9oz button mushrooms, thinly sliced
1 onion, thinly sliced
finely chopped parsley, to serve
1 tbsp Dijon mustard
400ml/14fl oz beef stock
salt and freshly ground black pepper
1 tbsp vegetable oil
25g/1oz butter
2 heaped tbsp sour cream or cr猫me fraiche
600g/1lb 5oz beef fillet, cut into 1cm/陆in cubes
1 tbsp brandy (optional)