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1
Butter a baking dish generously. Heat a sauté pan over a low flame and melt a portion of the butter. Add the onion half-moons and sweat without colour for 10–12 minutes, stirring occasionally, until completely soft and translucent. The onions are ready when no resistance remains and they carry a sweet, cooked aroma.
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2
Arrange a single layer of potato slices across the base of the buttered dish, overlapping slightly. Distribute a portion of the sweated onions evenly over the potatoes. Season with salt, white pepper, and a scattering of fresh thyme leaves. Repeat the layering — potato, onion, seasoning — pressing each layer firmly as it is built, until all the potato and onion is used. The surface should be level and compact to the touch.
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3
This is where the dish lives or dies: bring the chosen stock to a simmer and pour it steadily around the sides of the dish until it reaches three-quarters of the way up the potato layers. The topmost layer of potato must remain exposed; full submersion will prevent the crust from forming. Dot the surface evenly with the remaining butter.
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4
Place the dish in an oven set to 180 °C. Bake for 60–75 minutes. If the surface colours too rapidly, cover loosely with foil and remove it for the final 15 minutes. The dish is ready when the stock has been completely absorbed — no liquid should pool when the dish is tilted — the surface is deep golden and crisp, and the interior yields without resistance when pierced with a skewer.
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5
Remove from the oven and allow to settle for 5 minutes before serving. The interior should hold its layered structure when portioned, bound by the starch-thickened, concentrated stock. Serve directly from the dish alongside the roasted meat, so that any resting juices may mingle with the potatoes at the table.