Put the potatoes and vegetable stock in a large saucepan. Bring to the boil and cook, covered, for 12 minutes until tender but firm.
Meanwhile, preheat the oven to 230°C (450°F), Gas Mark 8 and heat the olive oil in a large, deep-sided, ovenproof frying pan over a medium-low heat. Add the onion and fry gently for 3–4 minutes, then add the garlic and cook for a further 3–4 minutes until softened and lightly coloured.
Stir into the onion mix the preserved lemon, roasted peppers, the spices, tomato purée, sherry vinegar and a little salt and pepper. Cook gently for 2 minutes.
Drain the cooked potatoes, reserving 400 ml (14 fl oz) of the liquid, then add the potatoes and reserved stock to the pan. Bring up to the boil, then put the pan in oven and bake for 14–16 minutes until golden. Remove from the oven and serve immediately with a crisp green salad.