Cut a slice off the top of each pepper and reserve. Remove the cores, seeds and membranes, and discard. Wash the peppers and pat dry with kitchen paper.
Heat 4 tablespoons of the oil in a large frying pan, add the peppers and sauté gently for 10 minutes, turning them frequently so that they soften and colour on all sides. Remove from the pan with a slotted spoon, and drain on kitchen paper.
To make the stuffing, drain off all but 2 tablespoons of oil from the pan, then add the onion and garlic, and seat very gently for about 15 minutes. Add the tomatoes and sweat gently to soften, stirring constantly. Increase the heat and cook rapidly to drive off the liquid – the mixture should be thick and pulpy.
Reduce the heat, and add the tomato purée, sugar. Season with salt and pepper, and simmer gently for 5 minutes. Remove the pan from the heat and stir in the chopped fresh coriander and the risotto rice. Spoon the stuffing into the peppers, dividing it equally between them.
Stand the peppers close together in a flameproof casserole dish. Sprinkle with the cinnamon, then the remaining 1 tablespoon oil. Put the reserved ‘lids’ on top.
Carefully pour 150ml/5fl oz water into the bottom of the pan, then bring to the boil. Reduce the heat, cover with a plate or saucer that just fits inside the rim of the dish, then place weights on top.
Simmer gently for 1 hour, then remove from the heat and leave to cool. Chill in the refrigerator overnight, with the weights still on top. Serve the stuffed peppers chilled, with garlic bread and a salad.