Remove outer leaves of cabbage (reserve for another use). Halve cabbage through the core, place in a steamer over a saucepan of boiling water and steam until the heart is easily pierced with a knife (15-20 minutes). Place in a colander over a bowl, put a saucer on top to weigh cabbage down slightly and refrigerate to cool (2-4 hours).
For miso dressing, cook ginger in the oil in a frying pan over medium-high heat until golden (5-7 minutes). Cool, then strain, reserving the oil. Pound the ginger to a paste with a mortar and pestle. Add the vinegar and miso paste. While mixing, slowly add the reserved oil to emulsify. Season to taste with salt and white pepper.
For nori powder, scatter orange rind on a ceramic plate lined with baking paper. Microwave on low at 1-minute intervals, checking to ensure the rind dries but does not burn (2-4 minutes). Set aside. Using tongs, gently wave the nori sheets about 20cm above an open flame to toast until crisp (4-6 seconds). Grind the nori in a spice grinder to a fine powder and combine in a bowl with the orange rind and ¼ tsp sea salt flakes.
To serve, preheat a char-grill pan or barbecue on high. Pull cabbage leaves apart and pat dry. Brush the leaves with a little oil and grill until charred (1-2 minutes). Transfer cabbage to a bowl and toss with 2 tbsp of dressing or to taste. Arrange the cabbage on a plate, drizzle with a little more dressing, scatter with diced nashi and season with nori powder.