Black garlic & beetroot beef Wellington

Season the beef generously, then rub the oil all over it. Melt the butter in a large frying pan over a high heat until foaming; sear the beef for 20-30 seconds all over. Transfer to a plate to rest. Set aside the pan and juices. When the beef has cooled down to nearly room temperature, spread the garlic paste all over it, then transfer to the fridge, uncovered.

Return the frying pan to a low-medium heat. Fry the shallot with a pinch of salt for 5-6 minutes without colouring. Stir in the thyme and breadcrumbs, cook for 1-2 minutes, then season, put on a plate and set aside. Wipe the pan clean and set aside.

Lightly flour a work surface. Roll out the pastry into a rectangle about 40x33cm (large enough to encase the beef). Square off the edges, sandwich between 2 sheets of baking parchment and chill until needed.

Meanwhile, bring a kettle to the boil. Put the spinach in a colander in the sink, then pour over the just-boiled water until wilted. Leave to cool for 5 minutes, then wrap in a clean cloth and squeeze out as much liquid as possible. Season, mix in the nutmeg and chop to a coarse paste; set aside. Put the beetroot in a food processor and pulse for 15-20 seconds to a coarse paste. Add the breadcrumb mixture, season and pulse again until combined.

On a large piece of baking parchment, arrange the speck in a rough 30x30cm square, letting the slices overlap by 1-2cm. Spread the spinach in a thin layer over the speck. Cover with another sheet of parchment and press gently all over with a rolling pin. Remove the top layer of parchment and spread the beetroot over the spinach. Repeat the parchment and rolling pin trick to ensure the layers are evenly compressed. It’s best (but not essential) to chill this for at least 30 minutes (or up to 2 hours) to firm it up before continuing with the next step.

When ready to assemble, remove the top layer of parchment, sit the cold beef in the middle of the beetroot layer, then lift up the base layer of parchment and carefully encase the beef. Press the beetroot/spinach paste to ensure even coverage and rearrange the speck as necessary. Pull the parchment tightly around the meat to compress everything.

Take the pastry out of the fridge. Remove the top layer of parchment and brush over a little beaten egg. Unwrap the beef and set it on the pastry so the longest length is parallel with one of the shortest lengths of pastry, 3-4cm in from the edge. Lift up that edge and tightly roll it around the beef, until the pastry meets and the meat is encased, leaving 2-3cm of excess pastry. Trim the excess, then fold the pastry ends over each other and under to seal. Slide the parchment with the Wellington onto a baking sheet and brush all over with ½ of the remaining egg. Chill for at least 2 hours (or up to 24 hours).

When ready to cook, heat the oven to 210ºC, gas mark 7. Brush the Wellington with more beaten egg and season with sea salt flakes.

Lightly oil a baking tray and transfer the Wellington onto it. Discard the parchment. Cook for 35-40 minutes, then rest for 20 minutes (don’t skip the resting time – it’s necessary here). Use a long, serrated knife to cut into slices 3-4cm thick and serve with buttered greens and potatoes, if liked. * Available from the meat service counter or to order through Waitrose Entertaining (5 days' notice)