Mix the flours, yeast and salt in a bowl. Add 255ml/9fl oz of water and, using your hands, mix to form a dough. Add a splash of the remaining water and continue to add it until all the flour has been incorporated and the edges of the bowl are clean (you may not need it all).
Knead for 5-10 minutes, tip the dough into a clean oiled bowl, cover with a tea towel and leave in a warm place until it at least doubles in size. This will take around three hours.
Once the dough has risen, tip out onto an oiled work surface. Using your hands push the dough out into a rectangular shape that is quite flat, like a pizza base. Using your fingers tips, push the surface of the dough to make dimples. Line a baking tray with parchment paper and transfer the dough to the tray. Prick the middle area of the dough with a docker or fork, leaving a rim untouched round the outside.
Leave in a warm place to prove for an hour.
Preheat the oven to 220C/425F/Gas 7. Bake the dough for approximately 30 minutes, or until golden-brown and cooked through.
For the lamb, in a large bowl mix the rosemary, garlic, lemon zest and oil. Add the lamb and coat each steak in the marinade. Leave in the fridge for a minimum of 30 minutes, but up to overnight.
For the dressing, in a bowl mix the dressing ingredients and season to taste.
Place a griddle pan on a high heat. Remove the lamb from the marinade and sprinkle over the crushed peppercorns. Season with a little salt. Fry the lamb in the hot pan for 2-3 minutes each side, or until cooked to your liking. Remove and leave to rest.
To serve, take a large serving board or plate. Place the trencher in the centre. Scatter the watercress, beans, spring onions and radishes over the top.
Slice the lamb on a slant and lay the pieces on top of the salad. Finally top with a little of the mustard dressing and serve at once.