Drain chickpeas, reserving liquid, and put chickpeas back in pot with 3 cups liquid, adding water if needed to make 3 cups (a surprising amount of liquid will get absorbed as the chickpeas cool). Grind saffron threads to a coarse powder in a spice grinder; set aside.
Heat oil in a medium frying pan over medium-high heat, add onion, and cook, stirring often, until softened and beginning to brown, 3 to 5 minutes. Stir in cumin and coriander and sizzle, stirring, 30 seconds. Stir in garlic, saffron, and salt, then pour into chickpeas. Add crushed tomatoes to chickpeas, stir, and simmer, covered, 10 minutes to meld flavors.