Using a pair of scissors cut the large leaves and stalks away from the cauliflower. (You can leave the little leaves close to the florets, they will go nice and crispy when roasted.) Turn the cauliflower upside down and, using a small paring knife, carefully cut a hollow in the middle of the stalk, so that it cooks evenly. Take a pan big enough to hold the cauliflower, half fill it with water from the kettle and bring it to a boil. Season the water with salt, then immerse the cauliflower and simmer for 6 minutes. Drain the water away, put the lid back on and leave the cauliflower to steam in the residual heat for 10 minutes. Meanwhile, cut the potatoes into 2cm pieces, leaving the skin on.