How to Make It
In a large pot, cover the chickpeas with water. Add the quartered onion and the thyme sprigs. Bring the chickpeas to a boil, then simmer them over moderate heat for 1 hour. Stir in 1 teaspoon of salt and simmer the chickpeas until they are tender, about 10 minutes longer. Discard the onion quarters and thyme sprigs.
Meanwhile, in a medium bowl, mash the garlic to a puree with 1 teaspoon of salt. Stir in the red wine vinegar, then gradually whisk in the olive oil.
Drain the chickpeas and add them to the bowl. Toss with the chopped celery, fennel, parsley and onion. Season with salt and pepper and serve warm.