To marinate the beef, mix the treacle with 100ml/3½fl oz of water. Put the beef in a container and cover with the treacle mixture. Leave to marinate for 24 hours, turning occasionally.
After 24 hours, remove the beef from the marinade and pat dry. Place the marinade in a pan and heat gently until the volume of liquid has reduced by half. Season with salt and pepper.
For the Yorkshire puddings, mix the eggs and milk together in a jug. Put the flour in a bowl and then slowly and gently whisk in the milk and egg mix. Don’t over whisk - lumps are okay. Rest the mixture at room temperature for at least four hours.
To cook the beef, preheat the oven to 55C/130F/Gas ¼ (or the lowest possible setting on your oven).
Heat a little oil in a large pan set over a high heat. When hot, add a knob of butter and fry the beef all over. Place the beef in a roasting tin and transfer to the bottom shelf of the oven for 45 minutes (or longer if you prefer your meat well cooked).
Remove the beef from the oven and brush with the reduced marinade. Cover and set aside to rest.
To make the roast potatoes, bring a large pan of salted water to the boil. Add the potatoes and cook until soft on the outside. Drain and let them steam dry on a wire rack placed over a roasting tin.
Preheat the oven to 200C/400F/Gas 6.
Add a little oil to a roasting tin and place in the oven to heat. When the oil is hot, add the dry potatoes and stir gently to coat in oil. Roast for 40 minutes, turning occasionally, until crisp and brown.
For the Yorkshire puddings, Add a little vegetable oil to each hole of a muffin tin or Yorkshire pudding tin and place in the oven (still at 200C/400F/Gas 6) to heat.
When the oil is hot and the Yorkshire pudding batter is rested, fill each hole of the muffin tin and cook in the oven for 15-20 minutes, or until risen and golden-brown.
Just before serving, wilt the spinach in a saucepan with a knob of butter. Season with salt and freshly ground black pepper.
To serve, heat the red wine sauce or gravy. Slice the beef and add to serving plates with the sauce, spinach, roasted potatoes and Yorkshire puddings.