Heat your oven to 200°C/Fan 180°C/Gas 6. Fill a large pan with boiling water, pop on a high heat to return the water to a simmer.

Trim the root and top 3cm off the leek. Halve the leek lengthways. Rinse under a cold tap to remove any grit. Finely slice the leek. Peel the onion and finely slice it. Peel the garlic cloves and finely chop or grate them. Trim any dry ends off the cavolo nero. Finely shred the leaves. Pop the cavolo nero into a bowl. Grate the cheddar into a bowl.

Pour 1 tbsp oil into a large pan and warm over a medium heat. Slide in the chopped leek, onion and garlic. Sprinkle in a little salt and pepper. Fry for 5 mins, stirring occasionally, till the veg have started to soften.

Tip the flour into the pan with the veg. Cook and stir for 2 mins to coat the veg and stop the flour catching on the bottom of the pan. Measure out 400ml of the milk. Pour a quarter of the milk into the pan and stir till the sauce becomes thick and smooth. Slowly pour in the remaining milk, stirring continuously. Bring to a simmer.

Meanwhile, scrub the potatoes. Slice as thinly as possible into rounds. Slide the sliced spuds into the pan of simmering water. Cook for 3-4 mins, till slightly softened but still holding their shape. Drain the potatoes well, then tip into a large ovenproof bowl.

Add the cheddar and cavolo nero leaves to the pan with the simmering milk sauce. Add a pinch of salt and pepper. Stir till the cheddar is melted. Pour the cheese sauce over the potatoes, and turn everything around till coated.

Use a slotted spoon to transfer the potatoes to an ovenproof dish. Slide into the oven and bake for 25 mins, till golden and bubbling.

While the potato bakes, pour 1 tbsp oil into a small bowl. Whisk in 1 tbsp balsamic vinegar and a little salt and pepper to form a dressing. Chop the cherry tomatoes in half and slide into a salad bowl. Scatter the peppery leaves on top.

When the gratin is cooked, scoop generous spoonfuls onto plates. Toss the tomato salad with the balsamic dressing and serve on the side.