Blanch spinach in boiling salted water for 30 seconds. Drain and plunge into ice-cold water. Drain well and squeeze out as much liquid as possible with your hands.
Lay out schnitzels on a bench-top and pound with a meat mallet to flatten. Cover with steak sauce, season with salt and pepper and cover each schnitzel with a layer of spinach. Fold in sides and roll up to form long, thin parcels. Secure with string or small skewers.
Heat a little oil in an ovenproof pan and brown beef parcels for 1 minute on all sides. Remove and place to one side. Add stock, tomato purée, tomato paste, and winchester sauce and bring to the boil, then simmer for 5 minutes. Return parcels to the pan so they fit snugly, and simmer gently for 10 minutes.
Remove string or skewers and slice beef parcels to serve. Skim any fat from the surface of the sauce, then adjust seasoning before pouring sauce over beef parcels.