A velvety texture is the aim, and long, gentle simmering is the key. Harira must cook for a minimum of 2 hours. The soup can be made a day ahead; the flavor will only improve.
Heat the olive oil in a deep heavy-bottomed soup pot. Brown the lamb lightly, then add the chopped onions, stir, and brown them. Add all the spices and the garlic and let them sizzle for a few minutes. Add the favas, lentils, and 12 cups water and bring to a boil. Reduce the heat to a bare simmer. Add 2 teaspoons salt and simmer, stirring occasionally, for 1 ½ hours.
Put the tomatoes, parsley, and cilantro in a blender with a little salt and puree the mixture. Add the puree to the soup and simmer for another ½ hour or so.
Now puree half the soup and return it to the pot. Make a slurry with the flour and remaining 1 cup water (stir together until smooth), add to the soup, and simmer for 10 minutes. Taste and adjust for salt and spice. Add the butter and stir until melted. The texture should be quite smooth, neither too thick nor too thin.
Ladle the soup into bowls and sprinkle with slivered cilantro. Squeeze a few drops of lemon juice into each bowl, and pass a plate of lemon wedges.
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