Photograph: Jonathan Gregson
Preheat the oven to 200°C, fan 180°C, gas 6. Trim the sprouts and peel off the outer leaves. Bring a pan of salted water to the boil, add the sprouts and cook until almost al dente; 5-8 minutes depending on their size. Drain and tip into a bowl of iced water. Leave for a few minutes, then drain and dry on kitchen paper. Halve the sprouts and arrange in an ovenproof dish.
For the crumble, gently soften the onion in the butter with the thyme for 8-10 minutes. Whiz the oatcakes and ½ teaspoon of sea salt flakes in a food processor until crumbs form. Mix with the onions and chestnuts.
Sprinkle the chestnut crumble over the sprouts and bake in the preheated oven for 20 minutes until golden.
Get ahead: prepare up to the end of step 2 a few hours ahead.