Pat the beef fillet with kitchen paper to remove any blood, then season. Heat the butter and oil in a large frying pan until very hot, then sear and turn the fillet with tongs for 8-10 mins until it’s well browned on all sides – hold it up to sear both ends, too. Set the beef aside on a tray to catch any juices. Take the pan off the heat, but don’t clean it.
Tip the spinach into a colander and pour over a kettle of boiling water to wilt it, then pour over cold water to cool it down. Squeeze the spinach as hard as you can to extract as much liquid as possible, then set aside. Lay a large sheet of cling film on your work surface. Overlap the pancetta or bacon on it in a row, then cover with another sheet of cling film. Use a rolling pin to roll it out to a thin layer. Remove the top sheet of cling film and scatter the spinach over the pancetta. Replace the cling film and roll again. Carefully peel away the top layer of cling film again, and sit the beef on top. Using the edge of the cling film, lift and roll the pancetta and spinach to encase the beef and roll everything into a tight sausage. For best results, place in the freezer for 30 mins to firm up – do not leave for longer or cooking times will be affected.
On a lightly floured surface, roll the pastry to a rectangle a little larger than your Good Food magazine. Trim the edges to neaten, then lightly brush the pastry with egg yolk. Carefully unwrap the beef parcel and lay in the middle of the pastry. Fold the shorter edges over the beef, then roll the whole thing around the fillet to encase. For a really neat finish, get another clean sheet of cling film and roll the Wellington into a tight sausage again. Chill for at least 30 mins, or up to 1 day.
Heat oven to 220C/200C fan/gas 7 and put a lightly oiled baking tray in it. Unwrap the Wellington, brush with egg yolk, then use the back of a knife to score a neat criss-cross or checkerboard pattern all over it. Sprinkle with flaky sea salt and transfer, sealed-side down, to the hot baking tray. Roast for 10 mins, then reduce the oven to 200C/180C fan/gas 6 and continue to cook for 25 mins for rare meat, 30 mins for medium rare, 35 mins for medium, and about 45 mins for well done, making sure the pastry doesn’t burn (cover it with foil if it starts to darken). Remove from the oven and immediately brush with more egg yolk. Leave to rest for 10 mins.
To make the gravy, heat the butter in the beef pan. Fry the shallot, thyme and bay leaf, scraping the crispy bits off the pan with a wooden spoon. Scatter over the flour and brown, then add the red wine and boil to a purple paste. Add the beef stock cube and any juice from the resting beef, and simmer for 5 mins. Pass through a sieve into a pan and season to taste. Use a sharp knife to carve the Wellington into 6 thick slices and serve with the gravy on the side.