- 4 tbsp rapeseed oil
- 2 carrots, chopped
- 2 celery stalks, chopped
- 500g/1lb 2oz roasted chicken bones (optional)
- 1 garlic bulb, crushed slightly with a rolling pin
- 100g/3½oz tomatoes, chopped
- 2 sprigs fresh thyme
- 2 tbsp chopped parsley stalks
- 3 large onions, chopped
- 2 tbsp chopped fresh coriander
- 3 large red peppers, thinly sliced
- 6-8 leftover roast potatoes, thickly sliced
- 50g/1¾oz butter, melted
- salt and freshly ground black pepper
- 1 tbsp oil
- 1 garlic clove, sliced
- 340g/11¾oz Portobello mushrooms, sliced
- 2 tbsp chopped fresh coriander
- For the casserole, preheat the oven to 180C/160C Fan/Gas 5.
- Heat 1 tablespoon of the oil in a heavy-based casserole over a medium heat. Add the carrots and celery and fry for 4-5 minutes, or until lightly browned.
- Add the chicken bones (if using; omit if cooking a vegetarian version of this dish) and fry for a further 2-3 minutes, stirring well.
- Add the garlic bulb, tomatoes, thyme and parsley stalks and stir again.
- Pour in 1.2 litres/2 pints water and bring the mixture to the boil. Reduce the heat then simmer for 25 minutes. Strain the resulting stock through a fine sieve, collecting the liquid in a clean saucepan. Simmer until its volume has reduced by about one-third. Keep warm.
- Meanwhile, heat another tablespoon of the oil in a frying pan over a medium heat. Add the onion and fry for 4-5 minutes, or until softened and lightly browned. Stir in the coriander, then season, to taste, with salt and freshly ground black pepper. Remove from the pan and set aside.
- Add another tablespoon of the oil to the frying pan, then fry the peppers until just softened (about 3-4 minutes). Season, to taste, with salt and freshly ground black pepper.
- Spoon half of the onion mixture into the bottom of a pie dish. Top with half of the peppers, then cover with a layer of the sliced roast potatoes, keeping the best shaped slices for the top layer.
- Repeat the layering process with the remaining onions and peppers, then pour enough of the warm stock into the pie dish to just cover the top layer of peppers.
- Decorate the top of the casserole with a layer of the remaining potato slices. Brush the potato slices all over with a mixture of the melted butter and the remaining tablespoon of oil.
- Cook the casserole in the oven for 20-30 minutes, or until the potatoes are crisp and golden-brown and the casserole is bubbling.
- Meanwhile, for the mushrooms, heat the oil and butter in a large frying pan over a medium heat. Add the garlic and fry for 1-2 minutes, or until softened.
- Add the mushrooms and fry for 2-3 minutes, or until softened and just coloured. Stir in the coriander just before serving.
- To serve, spoon the red-pepper-and-onion casserole onto serving plates. Serve the garlic-and-coriander mushrooms alongside, with any remaining warm roast potato slices.