Heat up the oil in a large casserole, add the meat, cumin, saffron, coriander, parsley, onion, celery and salt. Cook for 5 minutes.
Add the sparkling water and cook for 10 minutes.
Add the lentils and chickpeas, allow to cook for 50 minutes. Add the tomatoes and the tomato puree and simmer for 20 minutes.
Mix the flour with the water and whisk well for avoiding lumps.
Pour this mixture into the casserole, making sure that you stir all the time. Add the pasta and cook for 10 minutes more.
Serve hot in serving bowls, garnished with the lemon slices. lemon juice can be squeezed into the harira.