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1
Melt the lard in a heavy pan over low heat. Add the finely sliced onions with a pinch of salt and sweat, stirring occasionally, for 30–40 minutes. The onions are ready when they are deeply golden, entirely limp, and sweet to taste — no raw or sharp edge should remain.
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2
Add 1.5 litres of cold water to the onions. Season with salt and pepper. Bring to a simmer and cook uncovered for 20 minutes. The broth will be pale and light-bodied — this is correct; the cheese will provide all richness in the oven.
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3
Rub each slice of stale pain de campagne on both sides with the cut garlic clove. In a deep earthenware toupin or casserole, build alternating layers: a layer of bread, then an even layer of grated Cantal, then bread again, then Cantal, finishing with a generous, unbroken cheese layer on top. Press the layers firmly so they compact into a unified structure.
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4
This is where the dish lives or dies: ladle the hot onion broth slowly over the layered bread and cheese, pressing gently as you pour so the stale bread absorbs the liquid fully and the layers settle without floating apart. Every slice of bread must be saturated; dry pockets will not meld in the oven.
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5
Place the toupin, uncovered, in an oven at 200 °C for 20–25 minutes. The soup is ready when the surface is deeply gratinéed — bubbling at the edges, golden-brown across its entire top, and forming a crust that resists a spoon before giving way to the molten, bread-thickened, cheese-stringy interior beneath.
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6
Bring the toupin directly to the table. Scoop each portion to include crust, bread, melted cheese, and broth in equal measure. Those who observe the Auvergnat custom of faire chabrot may pour a splash of red wine into the last spoonfuls and drink from the tilted bowl.