Cheesy celery potato gratin

1. Heat 1 tbsp butter and the oil in a pan and, once it starts to bubble, add the bay leaves and onion. Cook over a medium heat for 2 minutes until the onion starts to soften. Add the celery and cook for 5 minutes. Add the potato, increase the heat to medium-high, and cook for 5 minutes until the vegetables start to colour. Stir through the salt, pepper and chilli flakes, then transfer to an ovenproof baking dish (about 25cm x 18cm x 5cm).

2. Preheat the oven to 200oC, gas mark 6. In a medium pan, heat 50g butter. Once it starts to bubble, add the flour and cook for a couple of minutes, then reduce the heat to low and gradually add the milk, whisking the whole time. Cook over a low heat for 2 minutes until the mixture thickens. Add the grated cheese and parsley; mix well. Pour over the celery in the dish.

3. Put the sourdough and walnuts in a food processor and pulse until crumbly but not too fine. Sprinkle over the cheese mixture, then bake for 35-40 minutes, until golden and bubbling. Once baked, let it stand for 10 minutes before serving with a leafy salad, if liked.