Prepare the potatoes: Slice the peeled potatoes into 3mm rounds using a mandoline or a very sharp knife. Place them in a large bowl of cold water as you go to prevent discolouration. The water will turn slightly cloudy — this is the starch being released, which helps the final dish hold together.

Infuse the cream base: In a large saucepan, combine the milk, double cream, minced garlic, bay leaves, and a sprig of thyme. Warm over medium heat until small bubbles appear around the edge — do not let it boil. The aroma will be fragrant and garlicky. Remove from the heat and let it steep for 10 minutes. Discard the bay leaves and thyme sprig.

Make the roux: In a separate large pan, melt the butter over medium heat until it foams and smells nutty. Whisk in the flour and cook for 1 minute, stirring constantly, until the mixture turns a pale golden colour. Slowly pour in the hot stock while whisking vigorously to prevent lumps. The sauce will thicken noticeably after about 2 minutes, becoming glossy and smooth.

Combine the sauce: Pour the infused cream mixture into the roux, whisking continuously. Add half the grated cheddar, half the Parmesan, the nutmeg, salt, and pepper. Stir until the cheese is fully melted and the sauce is velvety and coats the back of a spoon. Taste and adjust seasoning — it should be well-seasoned as potatoes absorb a lot of salt.

Layer the casserole: Preheat your oven to 180°C (160°C fan). Butter a 9x13 inch baking dish. Drain the potato slices and pat them completely dry with a clean tea towel. Spread a thin layer of the cheese sauce on the bottom of the dish. Arrange a third of the potato slices in overlapping rows, like shingles on a roof. Sprinkle with a few onion slices and a pinch of thyme leaves. Pour over a third of the remaining sauce. Repeat this process twice more, finishing with a generous layer of sauce on top.

Top and bake: Press down gently on the top layer with a spatula to ensure the sauce seeps between the layers. Cover tightly with aluminium foil and bake for 40 minutes. The dish will look puffed and the edges will be bubbling gently. Remove the foil, sprinkle the remaining cheddar and Parmesan over the top, and bake uncovered for a further 25–30 minutes. The top should be deep golden brown and bubbling vigorously, and a knife inserted into the centre should meet no resistance. The smell of toasted cheese and creamy potatoes will fill your kitchen.

Rest and serve: Remove from the oven and let the casserole rest on a wire rack for at least 15 minutes. This is non-negotiable — the casserole will be too loose to slice cleanly if you rush it. The surface will settle slightly, and the layers will firm up. Garnish with a few fresh thyme leaves and a final grating of Parmesan just before serving.