Place the peppercorns on a cutting board. Crush them by pressing on them with the bottom of a heavy pot, then mince them with a knife. Cut the tuna steak into 4 rectangular strips (the ends will be uneven, but that’s okay), then roll the strips lightly in the peppercorns. Sprinkle with salt.
Whisk together the mustard and soy sauce, then add some black pepper,