Mix the treacle with 100ml of water and marinate the beef for 24 hours, turning occasionally.
Remove beef from marinade and simmer the liquid in a pan until reduced by half, then season.
Whisk eggs and milk together, then slowly incorporate flour into the mixture, leaving it to rest for 4 hours.
Preheat the oven to 55°C.
Sear the beef in hot oil and butter, then roast on the bottom shelf for 45 minutes.
Brush the beef with reduced marinade and set aside to rest.
Boil potatoes in salted water until soft outside, then drain and steam dry on a wire rack.
Preheat the oven to 200°C.
Toss potatoes in hot oil in a tin and roast for 40 minutes until golden.
Add vegetable oil to a muffin tin and heat in the oven at 200°C.
Fill the muffin tin holes with batter and bake for 15-20 minutes until risen.
Wilt the spinach in a pan with butter and season with salt and pepper.
Slice the beef and serve with hot red wine sauce, spinach, potatoes, and Yorkshire puddings.