Warm olive oil in a large, cast-iron pot over medium heat. Add the chopped fennel and onion along with a big pinch of salt, and cook, stirring occasionally until soft (about 5 minutes). Stir in garlic, fennel seeds and saffron, and cook until fragrant (about 1 minute). Add diced tomatoes (with all of the juices), chickpeas, 1/2 cup water, a couple pinches of salt and several turns of black pepper. Let the sauce come to a boil, reduce heat to low, and simmer partially covered for 15 minutes, stirring occasionally. Season to taste with additional salt and pepper.