Chestnut Stuffed Roasted Onions

4 large, red onions
1x 400g tin of lentils – drained and rinsed
6 slices day-old wholemeal bread – rubbed through hands and broken into crumbs
250g fresh chestnuts – shell, peel and chop coarsely (or use vacuum packed whole chestnuts)
4 sticks celery – finely chopped
A few sage leaves – finely chopped
Sprig of thyme – leaves taken off stalk
Sprig of rosemary – leaves taken off stalk
Handful of fresh parsley – chopped
Olive oil
Salt and fresh ground black pepper to taste

To make stuffing (this can be made in advance and kept in fridge over night): Preheat the oven to 160C/325F/Gas Mark 3. Place the breadcrumbs in a shallow baking tin and bake in the oven until golden, approx 10-15 minutes. Take out and allow to cool. In a large saucepan, heat enough oil to cook the celery and herbs (except parsley), over a moderate heat, until the celery is soft. Add chestnuts and cook for a further minute. Add the lentils and breadcrumbs to the chestnut mix. Stir in the parsley, salt and pepper. Let stuffing cool completely.
For the onions, chop off tops and keep to one side. Peel, then chop off bottoms so they will sit on a flat surface. Use your hands to rub a little oil over the surface of the onions. Sprinkle with pepper and roast at 180C/350F/Gas Mark 4 for 20 minutes, or until they start to soften. Take out of the oven and when cool enough to handle, push out the centres, leaving a hollow shell to take stuffing. Chop up the removed onion centres and add to stuffing. Fill the hollow onions with the stuffing. Sit the onion tops back on and bake in the oven for a further 30 minutes.
Roll any remaining stuffing into balls and cook with the onions.
Serve as part of a Sunday roast dinner.