Preheat the oven to 180 degrees. In a large saucepan, melt the butter and sauté the leeks over a medium heat until soft but not mush – remember they will be cooked more in the oven. Add the garlic, thyme, crème fraiche and beans, season well and then pour the whole lot into a buttered gratin dish (I did say this was rich). Dot with the halved cherry tomatoes.

Mix together the Parmesan and breadcrumbs, with a little more thyme if you fancy and sprinkle on top of the beans. Then bake for 35-45 minutes until golden and bubbling.