Leek, Fennel, Garlic and Chickpea Mash

I first made this for my mother-in-law as part of our Christmas dinner, dolloped inside a tremendously large bowl of light and crisp puff pastry, and topped with a lightly charred, roasted cauliflower doused in onion gravy. The whole lunch was a resounding success, but months later, she still asks me for this recipe. So Sue, here it is, just for you.