Preheat the oven to 200°C/180°C fan/gas 6.
Put the cabbage in a large bowl. Mix the vinegar with the fennel seeds and honey or sugar and season. Add the boiling water, pour the mixture over the cabbage and toss to combine. Set aside for 30 minutes.
Finely chop most of the walnuts with the dried herbs, chilli flakes and breadcrumbs and season.
Put the fish on a baking tray and press the crumb over the top to coat, drizzle with the oil and bake in the oven for 10–12 minutes, until the crumb is golden and the fish is just cooked and opaque.
Set the peas and lettuce on two plates, top with the cabbage and fish. Scatter with the remaining walnuts and drizzle with oil.