Preheat the oven to 220°C/ 200°C (fan)/ 425°F/ Gas 7

Boil a kettle

Grate the potatoes (skin on)

Add the grated potato to a pot of boiling water and bring to the boil over a high heat with a pinch of salt

Once boiling, cook the potato for 5 min, then drain and leave to steam-dry in a sieve

Meanwhile, slice the leek[s] finely

Heat a large, wide-based pan (preferably non-stick) with 40g [80g] butter and a drizzle of olive oil

Once melted, add the leeks and cook for 3 min or until starting to soften

Add 1 tbsp [2 tbsp] flour to the softened leeks and cook for 1 min, stirring it with a wooden spoon

Add 300ml [600ml] milk and whisk for 3-5 min into a smooth sauce

Meanwhile, peel and grate the garlic into the sauce and add the bay leaf

Remove the skin from the mackerel and discard it, then break the mackerel up into different sized pieces, some small flakes and some larger chunks

Grate the cheddar

Once thickened, remove the sauce from the heat, and mix in the drained potato, mackerel and cheddar with a pinch of salt and pepper

Remove the bay leaf and transfer the mixture to an oven-proof dish (or individual ones)

Scatter the panko breadcrumbs over

Put the gratin[s] on a baking tray and into the oven for 10 min or until bubbling and golden on top

Tear the little gems roughly and add them to a bowl to serve

Chop the chives finely

For the dressing, combine the mustard, white wine vinegar, chives and 2 tbsp [4 tbsp] olive oil with salt and pepper

Dress the salad just before serving

Once golden, serve the gratin with the salad to the side (it will be very hot!)