For pickled cabbage, combine ingredients in a bowl, then refrigerate until lightly cured (24 hours).
For pumpernickel, preheat oven to 190C. Cook potato in a small saucepan of boiling water until tender (10-15 minutes), drain and mash, set aside 150gm (discard remaining). Combine polenta and 200ml water in a saucepan and stir continuously over low heat until thick (5 minutes). Add molasses, chocolate, butter, caraway and 2 tsp sea salt and stir to combine. Transfer to a large bowl and cool to room temperature (15-20 minutes). Meanwhile, combine yeast, potato and 50ml lukewarm water in a large bowl, stand until foamy (15-20 minutes), add wholemeal flour and 200gm rye flour and stir to combine, then knead until smooth. Transfer to a lightly oiled bowl, cover with plastic wrap and stand until doubled in size (1 hour). Knock back and knead lightly with remaining flour (add extra if mixture is too wet), then form into a 38cm-long log. Place on a baking tray dusted with polenta or lined with baking paper. Brush with eggwhite mixture and bake until hollow-sounding when tapped (50 minutes). Cool completely on a wire rack, then slice.
Preheat oven to 250C. Dry-roast spices until fragrant (1-2 minutes) and finely crush in a mortar and pestle. Combine with 1 tbsp sea salt flakes on a plate, add wagyu and roll to coat evenly. Heat oil in an ovenproof frying pan over medium heat, add wagyu and turn occasionally until browned (2-4 minutes), transfer to oven and turn occasionally until cooked rare (8-10 minutes). Set aside, cool to room temperature, then tightly wrap in plastic wrap to create a cylinder. Refrigerate until chilled (1 hour). Thinly slice and serve on pumpernickel topped with pickled cabbage and mayonnaise.