Add salt to sliced onions to help them break down and release moisture. Cook gently over medium heat until they are deeply caramelized, about 30 to 40 minutes. Ensure there are no black bits to avoid bitterness.
Use a mandolin to thinly slice the potatoes. This helps achieve uniform slices, which is important for even cooking. Do not wash the potatoes after slicing to preserve the starch, which will help the dish bind together.
Melt butter in a microwave for about 20 seconds. It's okay to leave the milk solids in the clarified butter as they will not burn at the temperature used for cooking.
In the base of an oven-safe pan, spread a layer of the caramelized onions. Add a thin layer of sliced potatoes over the onions. Drizzle some clarified butter and beef stock over the potato layer. Season with salt and pepper. Alternate the direction of the potato slices with each new layer to ensure an even arrangement. Continue layering potatoes, clarified butter, and beef stock, seasoning each layer with salt and pepper as you go.
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 to allow steam to escape. Place the cartouche on top of the layered potatoes to help with even cooking and caramelisation.
Cover the dish with a heavy plate to weigh down the potatoes. Bake in the oven at 120°C (250°F) for about 1 hour to 1.5 hours. Halfway through the cooking time, flip the plate to ensure even pressure on the potatoes.
After 45 minutes to 1 hour of cooking, remove the cartouche and plate. Raise the oven temperature to 130°C (265°F) to allow the beef stock to reduce and the potatoes to caramelise on top. 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. Serve with crispy edges and tender layers of potato, with the rich flavour of caramelised onions and thyme infused throughout.