How to Make It
In a saucepan, cover the chickpeas with 2 inches of water and bring to a boil. Reduce the heat to low and simmer until tender, 45 minutes. Drain.
In a bowl, crumble the saffron in the 2 tablespoons of water; steep for 10 minutes.
In a deep skillet, cook the onion and garlic in the butter and oil over moderate heat until golden, 8 minutes. Add the cumin, cinnamon stick and crushed red pepper and cook for 2 minutes. Add the saffron water, chickpeas, squash, potatoes, tomatoes, the quart of water and a large pinch of salt; bring to a boil, then simmer until the squash and potatoes are tender, 30 minutes.
Add the zucchini and the preserved lemon; simmer until the zucchini is tender, 5 minutes. Discard the cinnamon stick. Season the tagine with salt and pepper and stir in the cilantro. Serve in bowls with yogurt and harissa.