To make the red wine sauce, place the wine, beef stock and balsamic vinegar into a large saucepan. Bring to the boil and keep at a steady simmer until the sauce has reduced by half. Season with salt and freshly ground black pepper and leave to one side.
Preheat the oven to 200C/400F/Gas 6.
For the potato cakes, shape the potatoes into a barrel shape, then slice very thinly with a mandolin or sharp knife. Using a Yorkshire pudding tin, line each hole with a layer of potato, season with salt and freshly ground black pepper and pour over a little melted butter. Scatter the chopped prunes on top, then finish with the remaining potatoes and seasoning, pouring melted butter between each layer. Don't worry if the potato is higher than the holes - they will collapse during cooking.
Bake the potato cakes in the oven for 30 minutes or until cooked. Remove from the oven and allow to cool, then remove from the tins and set aside.
For the beef, season the fillets with pepper but not salt. To cook the fillets, heat the butter and oil in a frying pan and, once the butter is foaming, add the fillets. Colour well on one side before turning over and cooking for a further 5-6 minutes, or until cooked to your liking.
Remove the steaks from the pan and leave to rest in a warm place.
Add the spinach to the pan the steaks were cooked in and cook for long enough only to wilt, then remove from the pan and set aside. Sauté the onion, mushrooms, tomatoes, garlic and parsley in the same pan for a few minutes over a high heat to soften slightly. Pour in the red wine sauce, and a knob of butter to give the sauce a lovely gloss, and simmer for a minute or so. Check the seasoning.
To serve, divide the spinach between 4 plates, top with the warmed potato cakes and steak fillets, and finish by spooning the sauce and vegetables over the top.