Caramelise the onions by adding salt to sliced onions and cooking them gently over medium heat until deeply caramelized, about 30 to 40 minutes.

Thinly slice the potatoes using a mandolin to achieve uniform slices.

Clarify the butter by melting it in the microwave for about 20 seconds.

In an oven-safe pan, layer the caramelized onions, sliced potatoes, drizzle of clarified butter, and beef stock. Season each layer with salt and pepper.

Alternate the direction of the potato slices with each new layer.

After the last layer of potatoes, season again with salt and pepper and sprinkle fresh thyme leaves over the top.

Cut a piece of greaseproof paper to fit the pan, make a small hole in the centre, and place it on top of the layered potatoes to create a cartouche.

Cover the dish with a heavy plate and bake in the oven at 120°C (250°F) for about 1 hour to 1.5 hours, flipping the plate halfway through.

After 45 minutes to 1 hour, remove the cartouche and plate, and increase the oven temperature to 130°C (265°F). Bake for an additional 25 minutes or until the potatoes are golden and tender.

Before the final 5 minutes of cooking, brush the top of the potatoes with more clarified butter and a little beef stock to create a shiny glaze.

Add a final sprinkle of fresh thyme leaves for aroma.

Remove from the oven and let the dish rest for a few minutes before serving.