Heat a large, heavy-based pan for which you have a lid over a medium heat. Add half the butter, half the olive oil and the cider and, when it’s bubbling away, add the leeks.
Season well, then give the leeks a good shake, pop on the lid, reduce the heat and cook for eight to 10 minutes, until tender. You may need to stir them once or twice as they cook.
Tip the leeks and all their poaching juices into a bowl, then return the pan to a medium heat and add the remaining butter. When it’s bubbling, add the flour, stir well and cook for a minute.
Now add the hot stock, thyme, cream and mustard, bring to a simmer, then cook, stirring regularly, until the sauce is nice and thick. Fold in the leeks, their poaching liquid and the blue cheese, season to taste, then turn off the heat.
Heat the oven to 180C (160C fan)/350F/gas 4. Pour the gratin filling into a suitably sized oven-proof dish and level it off. Combine the breadcrumbs, porridge oats, broken walnuts and cheddar in a bowl with the remaining olive oil, then scatter over the top of the leeks.
Bake the gratin for 20 minutes, until the top is golden and deliciously tempting. Leave to stand for five to 10 minutes before serving.