Melt the butter in a small frying pan over a medium heat and add the leeks. As soon as they are sizzling, turn the heat down quite low and sweat gently, stirring often, for about 10 minutes until tender. Stir in the thyme, if using, and cream and cook for a minute or two longer until the cream is bubbling. Take off the heat and stir in two-thirds of the cheese. Add salt and pepper to taste.
Preheat the grill. Toast the bread lightly. Spread the leek mixture thickly over the bread and top with the remaining grated cheese. Grill until bubbling and golden, and serve straight away.
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