Preheat the oven to fan 200°C/Gas mark 7. Grease a 20cm ovenproof dish.

Mix everything together for the crumble topping in a bowl with your hands until it resembles breadcrumbs. You could also blitz this briefly in a food processor.

Fry the leek or onion in the butter or oil over a low heat for 10 minutes, adding the garlic for the final minute.

Meanwhile, in a medium pan on a high heat cook the celeriac slices for 5 minutes in strongly simmering salted water until just tender, then drain the excess water and let steam-dry in the pan for a few minutes. Remove the celeriac and set aside.

Back to the leeks. Add the chopped cavolo nero, 4 tablespoons of water and a pinch of salt and pepper, then pop the lid on and let steam for about 5 minutes.

Pop the celeriac pan back on the heat, heat up the butter for the sauce, then add the flour, whisking as you go for about 4 minutes. Slowly whisk in the milk until gently simmering and smooth, then add the nutmeg, herbs, mustard, most of the grated cheese and a little pepper, stir and take off the heat.

Return the celeriac slices to the pan and gently mix. Tongs work well here.

Make a layer of the creamy celeriac on the bottom of the ovenproof dish. Top with the cavolo nero and leeks or onion, then the rest of the creamy celeriac. Add the remaining cheese and the crumble topping and bake for 20 minutes or until golden on top. If you need to, pop it under the grill for the last 5 minutes to get the colour.

Waste Not – This is a great way to use up ends of cheeses and the last bits of packets of nuts and seeds for the crumble topping. You could also add in any leftover cooked meat or chunks of smoked fish just before it goes into the oven.