Step 1Heat the oil in a medium, heavy-bottom saucepan over medium heat. Add the onion and cook, stirring occasionally, or until it is soft and deep golden brown, about 25 minutes; as the onion starts to brown, reduce the heat and stir more often. Add the garlic and cook over medium-low heat, stirring often, for 1 minute. Transfer about one-third of the mixture (about 1/3 cup) to a small bowl; reserve it for garnish.
Step 2Add the chickpeas, broth and 2 cups water to the pan. Stir well to blend in the onion mixture and bring to a boil. Cover and cook over low heat for 1 1/2 hours. Add the lentils, turmeric, salt, pepper and 2 cups water. Return to a boil. Cover and simmer over low heat until the chickpeas and lentils are tender, about 30 minutes. At this point you can refrigerate the soup.
Step 3Boil 4 cups water in another medium saucepan. Add a pinch of salt and the linguine. Cook uncovered over high heat until just tender, checking about 2 minutes before the time on the package instructions. Drain the linguine, reserving the cooking liquid. You should have a scant 2 cups cooked pasta.
Step 4Meanwhile, mix 2 teaspoons each of the dill, cilantro and green onion; reserve as garnish.
Step 5Mix the remaining dill, cilantro and green onion and divide in two parts. Divide the spinach in two parts.
Step 6Reheat the soup to a simmer. Add one part of the spinach and one part of the herb mixture. Cook over medium heat, uncovered, for 10 minutes. Add one-half cup of the pasta cooking liquid and cook until the spinach is very tender and the soup takes on a spinach flavor, about 5 minutes. If the soup is too thick, add 1/4 cup pasta cooking liquid and return to a boil.
Step 7Just before serving, bring the soup to a simmer. Stir in the remaining spinach and add the cooked pasta. Cook uncovered until the spinach just wilts, about 1 minute, adding more pasta liquid by tablespoons if the soup is too thick. Stir in the second part of the herb mixture. Taste and adjust seasoning.
Step 8Reheat the onion and garlic mixture reserved for garnish in a very small skillet over medium heat until sizzling. Add the dried mint, stir and remove from heat.
Step 9Ladle the soup into a tureen or into bowls. Garnish each serving with a spoonful of kashk and with the onion-garlic mixture and the reserved herbs and green onion. Serve hot.