If using fresh, cut the cores from the tomatoes and criss-cross the bottoms. Put them in a bowl and cover with just-boiled water. After a minute, tip away the water and then peel the tomatoes, discarding the skin. Heat the oil in a medium saucepan until hot. Add the lamb to the oil and fry it briskly for about 3 minutes until browned. Lower the heat and add the coriander, ginger, celery and onion. Introduce the flour and then cook everything for 3 minutes, stirring regularly, until the onion has softened a little but not coloured.
While the vegetables cook, roughly but finely chop all the tomatoes, then add them to the saucepan with the tomato purée, saffron, 2 teaspoons salt and a good grinding of black pepper. (Be careful with the saffron, as it is popularly and wrongly thought to be just used for its colour when the good stuff in fact has a strong iodine-like taste.) Add the water and chickpeas and gently simmer for 30 minutes, with a lid on.
Break up the vermicelli in your hands and drop it in the soup to cook for a further 5–10 minutes. Stir the soup and you will see that the flour and starch from the vermicelli have thickened it slightly. Check the seasoning and serve hot in bowls, having passed over them a little lemon juice and oil.
Some like to add a dusting of cumin on top but, as Ibrahim says, not in the cooking. Warm white rolls are also good with harira.