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1
Peel, halve, and slice all onions to a uniform 2 mm. The raw volume will appear far greater than the pan can hold; this is correct — the mass reduces by 75% over the cook.
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2
Set a wide, heavy-based sauté pan over medium heat. Add the butter and the thread of neutral oil. Wait until the butter is fully melted and foaming at the edges before proceeding.
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3
Add all the sliced onions and stir immediately to coat every strand in the fat. Reduce the heat to the lowest possible setting. The pan must remain uncovered for the entire cook — steam trapped by a lid prevents caramelisation.
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4
Cook undisturbed except for periodic scraping of the pan base with a wooden spoon. At 15 minutes the onions should be fully translucent and collapsed; at 30 minutes they should be golden-blond. This is where the dish lives or dies: any increase in heat at this stage converts caramelising sugars to bitter char rather than the deep, complex sweetness the Trébons onion's natural sucrose is capable of producing. Maintain the lowest heat without exception.
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5
At 50 minutes, check that the pan base carries a uniform brown crust that lifts cleanly when scraped. Add the wine vinegar in a single splash. The deglaze is structural: it lifts the glaze and prevents the sweetness from becoming cloying. Stir to incorporate the lifted fond into the onions.
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6
Continue on lowest heat for a further 10 minutes. The finished confit is a deep amber-mahogany, jammy in texture but not sticky, with no pale or translucent pieces remaining. Finish with sea-mineral salt. The aroma is caramelised sugar, not sulphur; the taste is sweet, deep, and complex with a long finish balanced by the vinegar.