Tuna Steak with Béarnaise Sauce

Mince the shallots, roughly chop the tarragon stems for the reduction, and finely chop the fresh tarragon leaves and chervil for finishing the sauce. Clarify the butter by melting it gently over low heat, skimming the foam, and pouring off the clear golden fat, discarding the milky solids. Keep the clarified butter warm.

Combine the white wine, white wine vinegar, minced shallots, roughly chopped tarragon stems, and crushed peppercorns in a small saucepan. Bring to a simmer over medium heat and reduce until approximately 3 tablespoons of liquid remain, about 8–10 minutes. Strain through a fine sieve into a heatproof bowl, pressing the solids; discard the solids.

Set the heatproof bowl over a saucepan of barely simmering water, ensuring the base does not touch the water. Add the 3 egg yolks to the cooled reduction. Use pasteurised egg yolks if serving anyone pregnant, elderly, very young or immunocompromised — a béarnaise is only lightly cooked and does not reach a temperature that reliably pasteurises egg.

Whisk continuously for 4–5 minutes until the mixture is pale, thick, and holds a ribbon — the whisk should leave a trail on the bottom of the bowl. Keep the water at a bare simmer, never a boil: the yolks scramble above roughly 70°C, so if the sabayon looks like it is thickening unevenly or catching at the edges, lift the bowl off the heat for a few seconds while whisking.

Remove the bowl from the heat. Begin adding the warm (not hot) clarified butter one teaspoon at a time, whisking constantly, until the emulsion is established and thickened. Once stable, pour the remaining butter in a thin, steady stream, whisking continuously, until all the butter is incorporated and the sauce is thick enough to coat a spoon. If the sauce ever looks thin, greasy, or starts to split, stop pouring immediately and whisk in a teaspoon of cold water or an extra yolk to bring it back together before continuing.

Fold in the finely chopped tarragon leaves and chervil. Season with lemon juice, fine sea salt, and white pepper. Keep the sauce warm over the hot water bath — off the heat — while you cook the tuna, whisking occasionally so it does not split.

Pat the tuna steaks completely dry with kitchen paper. Season all surfaces generously with flaky salt and cracked black pepper.

Heat a cast-iron skillet over the highest heat for 3–4 minutes until it begins to smoke. Add the neutral oil and swirl to coat.

Place the tuna steaks in the pan without moving them and sear for 90 seconds until a deep mahogany crust forms on the first side.

Flip the steaks and sear for a further 90 seconds on the second side. Use a meat thermometer to check the internal temperature — for rare, pull at 46°C; for medium-rare, pull at 52°C. Tuna's recommended safe internal temperature is 63°C, though traditionally this dish is served medium-rare; either is safe, so cook to whichever doneness you and your guests prefer.

Transfer the tuna to a warm plate and rest for 2 minutes. Spoon the béarnaise generously over or alongside each steak and serve immediately.