Preheat a large frying pan on a high heat. Rub the beef all over with sea salt and black pepper. Pour a good lug of oil into the pan, then add the beef, 1 knob of butter and 1 sprig of rosemary.
Sear the beef for 4 minutes in total, turning regularly with tongs, then remove to a plate.
Wipe out the pan and return to a medium heat. Peel the onion and garlic, then very finely chop with the mushrooms and put into the pan with the remaining knob of butter and another lug of oil.
Strip in the rest of the rosemary leaves and cook for 15 minutes, or until soft and starting to caramelise, stirring regularly.
Toss the livers into the pan and cook for another few minutes, then tip the contents onto a large board and drizzle with the truffle oil (if using).
Finely chop it all by hand with a big knife, to a rustic, spreadable consistency. Taste and season to perfection, then stir in the breadcrumbs
Preheat the oven to 210°C/425°F/gas 7.
On a flour-dusted surface, roll out the pastry to 30cm x 40cm. With one of the longer edges in front of you, spread the mushroom pâté over the pastry, leaving a 5cm gap at either end and at the edge furthest away from you – eggwash these edges.
Sit the beef on the pâté then, starting with the edge nearest to you, snugly wrap the pastry around the beef, pinching the ends to seal.
Transfer the Wellington to a large baking tray lined with greaseproof paper, with the pastry seal at the base, and brush all over with eggwash (you can prep to this stage, then refrigerate until needed – just get it out 1½ hours before cooking so it’s not fridge-cold).
When you’re ready to cook, heat the tray on the hob for a couple of minutes to start crisping up the base, then transfer to the oven and cook for 40 minutes for blushing, juicy beef – the two end portions will be more cooked, but usually some people prefer that.
For the gravy, peel and roughly chop the onions and put into a large pan on a medium heat with a lug of oil and the thyme leaves. Cook for 20 minutes, stirring occasionally, then stir in the jam and simmer until shiny and quite dark.
Add the Madeira, flame with a match, cook away, then stir in the mustard and flour. Simmer to the consistency you like, then blend with a stick blender and pass through a sieve, or leave chunky.
Once cooked, rest the Wellington for 5 minutes, then serve in 2cm-thick slices with the gravy and steamed greens.