Place the cauliflower and leek in a shallow, flameproof casserole dish and pour in the stock. Cover and simmer for 5 minutes, then pour away half the stock.
Transfer 3 tablespoons of the remaining stock to a cup and mix in the cornflour until smooth. Stir in the crème fraîche, then stir the mixture into the pan along with half the cheese. Cook for 1 minute, then season to taste.
Sprinkle the remaining cheese over the vegetables and place in a preheated oven, 200°C (400°F), Gas Mark 6, for 15–20 minutes until golden and bubbling. Serve with crusty bread.