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1
Set a dry comal or heavy cast-iron skillet over medium-high heat. Place the onion halves cut-side down directly on the dry surface — no oil. Place the whole habanero alongside. Char without moving until the cut faces are deeply blackened and the outer layers are scorched; turn and continue until the skins are charred on all sides and the flesh has yielded and softened throughout. The test: a skewer meets no resistance at the centre of the thickest half.
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2
While the onions and habanero are still on the comal and hot, transfer the habanero to a small bowl. Slice or tear it open and stir it into the lime juice with the pinch of salt to form the marinade. The habanero's char moderates its raw heat and adds complexity to the marinade.
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3
Transfer the charred onion halves immediately to a shallow dish while still hot. Pour the habanero-lime marinade over them. The test: the warm onion visibly relaxes and begins to absorb the marinade at once — the cut surfaces will deepen in colour as the liquid penetrates.
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4
Allow to rest and cool to room temperature. Do not refrigerate before serving. The test: the onions are neither steaming nor cold to the touch — room temperature, at which the full balance of sweet, smoky, and floral heat is perceptible on the palate.