Bacalao al pil-pil recipe: Bilbao's trembling cod sauce

Bacalao al pil-pil turns three ingredients, salt cod, olive oil and garlic, into a thick golden sauce with no thickener at all, bound entirely by the gelatin the fish releases as it cooks.

The dish is said to date to the 19th-century siege of Bilbao during the Carlist wars, when cooks had little beyond salt cod, oil and garlic to work with. The technique survived because it is genuinely clever: done right, the sauce clings to the fish like a loose mayonnaise, with nothing added to make it so.

Ingredients

The sauce clings to the fish like a loose mayonnaise, with nothing added to make it so.

• 4 skin-on salt cod loins, desalted (about 600g total)
• 500ml extra virgin olive oil
• 4 garlic cloves, thinly sliced
• 1 dried guindilla chilli, crumbled (or a pinch of cayenne)
• 2 slices stale bread (to protect the garlic as it fries)

Method

1. The night before, warm the olive oil in a wide pot and add the garlic and bread. Fry gently until the garlic turns pale gold, pulling the bread out first so it doesn't scorch. Let the oil cool and infuse overnight, then strain out the garlic and bread.
2. Trim the cod loins to an even thickness and lay them skin side up in a wide, shallow pot. Pour over the garlic oil.
3. Heat gently until the oil just starts to shimmer, around 70°C, well short of a simmer. Chef Diego Sánchez, who runs Amets in the Basque Country, says this is the step most home cooks get wrong: any hotter and the fish tightens before it releases its gelatin.
4. Cook for 15-20 minutes in short bursts, removing the pot from the heat for a few minutes between each, basting the fish gently with the oil.
5. Take the pot fully off the heat and swirl it in slow, steady circles. As the oil cools slightly, the released gelatin binds with it, thickening into a pale, trembling sauce, the pil-pil.
6. Return briefly to low heat until the sauce just bubbles, then serve at once with the fried garlic and guindilla scattered over the top.

Why it never boils

Pil-pil is really an emulsion, the same principle as mayonnaise, except the egg yolk is replaced by collagen dissolved out of the cod's skin. Too much heat cooks that collagen solid instead of dissolving it, which is why every step stays well under a simmer.

Serve with a glass of txakoli, the sharp, lightly sparkling Basque white built for exactly this kind of oily, garlicky fish. Leftover sauce is worth keeping: spooned over boiled potatoes, it is most of a second meal. For a related boat-to-pot Basque classic, see AMUSE's marmitako recipe.

Can I use fresh cod instead of salt cod? Fresh cod works, but it releases less gelatin, so salt it lightly for two hours first and pat it dry; the sauce will still bind, just a little thinner.