Heat a large frying pan with a glug of olive oil as hot as you dare. Salt and pepper the beef all over and sear for 4 minutes in total, turning regularly, remove to a plate. Chill.
Return pan to a medium heat and add another glug of olive oil. Sauté the shallots, garlic, mushrooms and the rosemary leaves and cook until soft and starting to caramelise, stirring regularly. Allow to cool a bit and tip into a blender with the foie gras and the truffle oil (if used). Blend to a rustic, spreadable consistency. Taste and season to perfection, then add sufficient breadcrumbs to retain any residual liquid (you can use pancakes to line the pastry and absorb the juices, I prefer using breadcrumbs like this). Chill.
On a flour-dusted surface, roll out the pastry so that it will cover the beef (and pâté) with 5 cm overlap at the back and either end. 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). Get it out 2 hours before serving so it's not fridge-cold.
50 minutes before serving have the oven preheated to 200°C/180°C (Fan)/Reg 7 (Gas). 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 30 minutes for rare, 40 for medium rare or 50 for medium - the two end portions will be more cooked, but usually some people prefer that. Make the gravy while it is cooking (or beforehand - it can be reheated.)
GRAVY
Sauté the gravy shallots on a medium heat with a glug of oil and the thyme leaves for 20 minutes, stirring occasionally, then stir in the jam and simmer until shiny and quite dark. Add the Madeira/Sherry, cook another few minutes, add the (not too hot) stock and blend in the mustard and flour with a stick blender.
Rest the Wellington for 5 minutes, then serve in 2cm-thick slices with the gravy, roast potatoes and a green vegetable.