Heat 25g/1oz of the butter in a large frying pan over a medium heat. Add the chopped shallots and garlic and fry for 2-3 minutes, or until softened but not coloured.
Add the mushrooms and red wine and bring to the boil. Boil for 3-4 minutes, or until the volume of liquid has reduced by half.
Add the stock, return the mixture to a simmer and simmer for 8-10 minutes, or until the volume of liquid has reduced by half again and slightly thickened.
Meanwhile, heat half of the vegetable oil and another 25g/1oz of the butter in a large frying pan over a high heat. Add the potatoes, lightly crushed garlic and thyme. Fry, stirring occasionally, until the potatoes are just coloured on each side, then add the cavolo nero and stir to combine. Pour in 100ml/3½fl oz water, then bring the mixture to a simmer, cover the pan, and simmer gently for 8-10 minutes, or until the potatoes are tender. Remove from the heat and season, to taste, with salt and freshly ground black pepper.
Meanwhile, rub the steaks all over with the remaining vegetable oil and season well. Heat a griddle pan over a high heat. When the pan is hot, add the steaks and fry for 3-4 minutes on each side, or until cooked to your liking. Transfer to a warm plate and set aside to rest for 5-10 minutes.
Season the sauce, to taste, then stir through the parsley and the remaining butter.
To serve, pile the potatoes onto serving plates and place one steak alongside each portion. Spoon over the sauce bordelaise.