Shooter's Sandwich

Method

1. Saw the top third off the 1 crusty loaf with a bread knife and keep it as a lid. Pull out the soft crumb from both halves with your fingers, leaving a shell about 1 cm thick all round — it must be sturdy enough to hold the juices without collapsing.
2. Melt 10 g of the butter in a heavy frying pan over a medium-high heat and add the 80 g of sliced mushrooms with the 1 finely chopped shallot. Fry 6-8 minutes, stirring, until the mushrooms have given up their water and are browned and dry. Season with a pinch of the salt and pepper and tip onto a plate.
3. Pat the 150 g sirloin dry and season both sides hard with most of the remaining 0.5 tsp of salt and the 0.25 tsp of pepper. Return the pan to a high heat, add the last 5 g of butter and, when it foams, sear the steak 2-3 minutes per side for rare to medium-rare (it will firm up under pressing, so err on the rare side). Lift onto a board and rest 5 minutes; keep every drop of pan juice.
4. Spread 1 tsp of English mustard over the inside of the hollowed base and lid. Lay the whole rested steak in the cavity, trimming it to fit and tucking the offcuts down the sides, then press the mushrooms over and around it in a firm layer.