Preheat the oven to 180C/350F/gas mark 4. Mix the breadcrumbs, nuts, garlic and herbs together in a bowl. Rub in the butter or process together in a food processor, then stir in the cheese. Press the mixture firmly into the base of an 20cm flan dish. Bake for 20 minutes until golden.
2 For the sauce, melt the butter in a frying pan and fry the leeks for about 10 minutes until soft. Add the sage and seasoning. Finally, add the creme fraiche until thick and bubbling.
3 Spread the leek sauce over the cooked croustade. Bake for a further 20 minutes. Serve.