Boil red wine and beef stock then simmer until reduced to 50ml.
Fry diced onion in a knob of butter for 2 minutes until soft.
Mix remaining butter, vinegar, reduction, and parsley with the fried onion and season.
Season steaks with salt and pepper and rub with olive oil.
Grill steaks for 3 minutes per side and rest for 4 minutes with onion butter on top.
Heat deep-frying oil to 190°C.
Prepare a plate of seasoned flour and a bowl of beaten eggs.
Dredge onion rings in flour, dip in egg, and fry for 2 minutes until golden.
Slice steak and serve topped with more onion butter and the fried onion rings.