Beef Strogaanoff


600g beef fillet
25g butter
1 onion, thinly sliced
250g button mushrooms, thinly sliced
1 tbsp Dijon mustard
400ml beef stock
1 tbsp vegetable oil
2 heaped tbsp soured cream or crème fraiche
1 tbsp (or more) brandy (optional)
squeeze of lemon (optional)
finely chopped parsley, to serve
flaked sea salt
freshly ground black pepper