Season the beef with salt and freshly ground black pepper.
Heat a frying pan until hot, add the oil and some of the butter, seal the beef on each side for three minutes then remove from the heat and set aside to rest.
Heat a sauté pan until hot; add some more of the butter and the onion, cut side down. Fry until the bottom of the onion is very browned then add the Madeira and cook until reduced by half.
Add the red wine, reduce again then add the stock and cook once more until reduced by half. Strain into a clean pan then whisk in 25g/1oz of the butter and season with salt and freshly ground black pepper. Keep warm.
Heat a frying pan until hot, add a little more butter and the spinach, cook until the spinach has wilted down then season with a little salt, freshly ground black pepper and nutmeg. Drain the spinach onto kitchen paper.
Heat the last of the butter in a clean frying pan until foaming then add the brioche slices and cook on each side until golden brown. Remove from the pan and drain onto kitchen paper.
To serve, lay the brioche onto the centre of the plate then spread the duck liver pâté over the top. Place a pile of spinach on top then the rested steaks. Finish with a ladle of Madeira sauce over the top.