Melt the butter in a saucepan over a low heat. Add the garlic and thyme and cook gently for 2–3 minutes. Don’t let the garlic brown – just allow it to soften. Pour in the cider, bring to the boil and bubble for 3 minutes to burn off the alcohol and reduce a little.
Meanwhile, arrange the leeks in a single layer, cut-side up, in an ovenproof dish measuring 20 × 20cm. Really pack them in, leaving as few gaps as possible. You might need to rearrange smaller leeks here and there to plug any gaps, like a leek jigsaw.
Pour the cidery liquid over the leeks and transfer to the oven to bake for 35 minutes while you get on with the crumble.
Combine the flour and oats in a mixing bowl. Add the cubes of butter and rub them into the flour with your fingers. Think light and swift: you will end up with a few dabs of butter here and there, but it should be mostly mixed in to make a floury rubble.
Add the Cheddar and chestnuts, along with a big pinch of salt and mix everything together.
Once the cooking time is up on the leeks they should be soft, but not dissolving, and swimming in a rich juice
Remove the dish from the oven and pile the crumble evenly on top, then bake for another 35 minutes, by which time the crumble will be golden and crunchy on top and bubbling at the edges. Remove from the oven and serve.