Heat the oven to 200°C (Gas Mark 6 / 400°F). Place the whole peppers on a baking tray, and roast for around 30 minutes, until soft and slightly charred. Remove from the oven, and place on a plate. Cover with cling film.Meanwhile, heat the oil in a saucepan and cook the onion and garlic over a medium-low heat for 10 minutes, until soft and translucent. Set aside until needed.When the peppers are cool enough to handle, remove them from under the clingfilm, and remove the stalk, seeds and skins (they should peel off easily). Chop the flesh into small pieces, and add to the onion and garlic. Return the pan to a medium heat, and mix well to break down the pepper a little. Add the stock and tomato pureé, and season well with black pepper. Bring to a simmer, and add the orzo. Simmer gently for around 10 minutes, stirring regularly to ensure the orzo doesn't settle and catch on the bottom of the pan.When the orzo is nearly cooked, add the spinach and parsley. Cook for a further couple of minutes, and then serve, topped with grated cheese if desired.