Season the diced lamb. Heat 2 tablespoons of the oil in a large, flameproof casserole, add half the lamb and fry over a high heat until nicely browned all over. Spoon onto a plate and repeat with the rest of the lamb (you shouldn’t need any more oil for the second batch). Add the chorizo to the casserole and fry briefly until lightly golden. Scoop out to the plate.

Preheat the oven to 150°C, fan 130°C, gas 2. Add the remaining tablespoon of olive oil to the casserole with the onions and garlic and fry over a medium heat for 8-10 minutes until soft and lightly golden. Add the carrots, leek and celery and fry for 5 minutes more. Stir in the smoked paprika, tomato purée and flour and cook for 1-2 minutes, stirring. Gradually add the stock, stirring until smooth. Bring to a simmer and add the thyme, bay leaves, lamb, chorizo and seasoning. Cover with a tight-fitting lid and cook in the oven for 11⁄2 hours, stirring occasionally.

Remove the lid from the casserole and cook uncovered in the oven for another 30 minutes to reduce and thicken the sauce, by which time the lamb should be meltingly tender and just falling apart. Meanwhile, cut the potatoes into large chunks, put into a large pan of cold salted water and bring to the boil. Cook for 20 minutes until tender then drain into a colander and leave to steam for a couple of minutes.

Melt the butter in the used potato pan, cook the garlic for 1 minute, then mash with the potatoes and parsley, off the heat. Season to taste.

Put a baking tray into the oven and heat to 200°C, fan 180°C, gas 6. Season the lamb mixture to taste and spoon into a deep 24cm x 30cm ovenproof baking dish. Spoon over the mashed potatoes, making sure they make a good seal with the edge of the dish. Lightly rough up the surface with the back of a fork and scatter over the grated cheese.

Bake for 35-40 minutes until bubbling hot and golden brown.