Preheat the oven to 180°C fan/200°C/ gas 6. Put the butter and garlic into a roasting tin and pop into the oven to melt while you get on with slicing the leeks.
Mix the sliced leeks with the melted garlic butter, season well with the sea salt and black pepper, then return to the oven to roast for 20 minutes.
After 20 minutes, stir through the Puy lentils, creme fraiche and another good scatter of sea salt, then top with the feta cheese and breadcrumbs. Drizzle with the olive oil, then return to the oven for a further 20-25 minutes, until golden brown on top.
Serve the gratin hot, with a mustard or balsamic dressed green salad alongside.