On Crete, chickpeas are commonly flavored with Seville orange, a pleasingly bitter variety grown locally. Orange zest along with lemon juice makes a good substitute. A little mustard deepens the flavor.
Place chickpeas in a large bowl with enough cold water to cover by 3 inches. Let soak for 8 to 24 hours.
Preheat oven to 400 degrees F.
Rinse the chickpeas. Transfer to a medium bowl and toss with baking soda.
Heat oil in a large ovenproof pot over medium heat. Add onions and cook, stirring occasionally, until soft, about 4 minutes. Stir in the chickpeas, Aleppo (or crushed red pepper) and salt; cook for 30 seconds. Add broth, celery leaves and orange zest. Bring to a boil over high heat, then remove from heat. Cover the pot with a double layer of foil and put the lid on.
Reduce oven temperature to 300 degrees . Bake the chickpeas until very tender, about 4 hours. Stir in squash, lemon juice and mustard; bake until the squash is tender, about 1 hour more.
Serve drizzled with more oil and topped with more celery leaves, if desired.