Peel the onion and roughly chop with the mushrooms. Put a 30cm non-stick frying pan on a high heat. Season the steak with sea salt and plenty of black pepper and rub with 1 teaspoon of olive oil. Turning with tongs, sear the steak all over for 2 minutes in total, then remove to a plate. Return the pan to a medium heat with the onion and mushrooms. Strip in the thyme. Cook for 15 minutes, or until soft, stirring regularly. Blitz in a food processor until spreadable, season to perfection, and remove. Blitz 1 egg, the flour, spinach, a pinch of salt and 1 mug of water in the processor until smooth. Put your pan back on a medium heat, rub with oil, then pour in a thin layer of batter. Cook for 1 minute on each side without colour. Tip on to a plate to cool. Cover the leftover batter and chill for breakfast or brunch.
Preheat the oven to 220°C. Sit your pancake on a large sheet of clingfilm. Evenly spread over the mushroom pâté. Place the steak in the centre, then gather up the clingfilm and twist into a parcel. Sit the wrapped steak (clingfilm discarded) on the pastry, 2cm from one side. Eggwash all the pastry, then fold and mould the excess over the wrapped steak, leaving a pastry border around it. Trim to 2cm, pinch the edges to seal, eggwash, and decorate with the trimmings, if you like. Cook on the bottom of the oven for 25 minutes, or until the pastry is golden on top and crispy underneath, for blushing, juicy beef. Rest for 2 minutes, then serve.