Roughly chop the leek trimmings, place in a pan and pour over the milk. Add the parsley stalks and bay leaves. Place the pan on a medium heat, bring up to the simmer and immediately turn off the heat. Let the milk infuse for a while.
Place another large pan on a medium heat. Add 25g (1oz) of the butter and, when it’s melted and bubbling away, add the leeks, cep mushrooms and garlic and season lightly with salt and pepper. Add 3 - 4 tablespoons of water and place a lid on the pan. Let the leeks steam gently for 10 – 12 minutes or until they are tender.
Heat the oven to 200°C/180°C fan/400°F/gas mark 6.
Melt the remaining butter in another large pan over a medium heat. When it’s melted and bubbling, stir in the flour. Reduce the heat to medium–low and cook gently for a minute or so, then pour half the infused milk through a sieve into the pan. Use a whisk to mix the sauce vigorously, then sieve in the remaining milk and whisk again. Add all but a handful of the cheese to the sauce along with the mustard, thyme leaves and chopped parsley and stir it in well. Taste, and season with salt and plenty of black pepper. Finally fold the steamed buttery leeks and mushrooms into the sauce.
Spoon out the mixture into a large ovenproof dish or roasting tin and give it a shake
to level it out. Scatter over the breadcrumbs and the remaining cheese and bake for 12–15 minutes, or until the sauce is bubbling away and the breadcrumbs are golden, crisp and crunchy.