Celery Gratin

1 celery head, trimmed
25g butter
1/2 onion, thinly sliced
2 bay leaves
100g breadcrumbs
50g walnuts, roughly chopped
40ml white wine
100ml vegetable or chicken stock
100ml double cream
30g grated Parmesan or Gruyere

1.  Cut any thick celery stalks in half, trim all of it into thumb-size lengths, then wash and leave wet. Melt half the butter in a large frying pan, then add the celery, onion and bay leaves. Season, cover, then cook over a medium heat for about 30 mins, stirring occasionally to stop the onions catching.
2.  Meanwhile, prepare the breadcrumbs. Melt the remaining butter in a separate pan, then toss in the crumbs and walnuts, stirring often until lightly golden and toasted. Set aside.
3.  Heat grill to medium. When the celery is tender, turn the heat right up, pour in the wine and stock, then reduce by two-thirds. Pour in the cream, then reduce for a final few mins until you have a syrupy sauce. Check seasoning, tip into an ovenproof dish, then scatter with the breadcrumbs and Parmesan. Grill for 2-3 mins, until the sauce bubbles. Let it sit for 5 mins before serving.