Grilled Portobello with Béarnaise Sauce

Pat the portobello caps dry on both sides with paper towels. Sprinkle evenly with flaky sea salt on both sides and place gill-side up on a wire rack set over a baking tray.

Leave the salted mushrooms uncovered at room temperature for 30 minutes to draw out excess moisture, then pat thoroughly dry again with paper towels. Brush both sides lightly with olive oil and season with cracked black pepper.

Meanwhile, combine the white wine, white wine vinegar, minced shallots, tarragon stems, and crushed peppercorns in a small saucepan over medium heat. Simmer until the liquid is reduced to approximately 2 tablespoons of syrupy liquid, about 8–10 minutes. Strain through a fine-mesh sieve, pressing the solids, and allow the reduction to cool to room temperature.

Place the strained reduction and egg yolks in a heatproof bowl set over a saucepan of barely simmering water. 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. Ensure the bowl does not touch the water. Whisk continuously until the mixture is pale, thick, and leaves a ribbon trail when the whisk is lifted, about 4–5 minutes — this sabayon stage should stay below 70°C throughout; if it looks like it is scrambling at the edges, lift the bowl off the heat and keep whisking to cool it before continuing.

Remove the bowl from the heat and begin adding the cold butter cubes one at a time, whisking constantly and waiting until each piece is fully emulsified before adding the next. Continue until all butter is incorporated and the sauce is thick, glossy, and coats the back of a spoon.

Stir in the chopped tarragon, chervil, and lemon juice. Season with salt and white pepper. Keep warm by setting the bowl back over the hot (not simmering) water off the heat, whisking occasionally — do not let the sauce overheat or it will split.

Heat a ridged cast-iron grill pan over high heat until it begins to smoke. Place the portobello caps gill-side down and grill without moving for 4 minutes, until deep char marks form and the caps release naturally from the pan.

Flip the caps and grill on the dome side for 3 minutes until the mushrooms are tender through to the centre and any remaining moisture has evaporated. Transfer to a warm plate and rest for 2 minutes.

While the mushrooms rest, melt 1 tablespoon of butter in a wide skillet over medium heat until foaming. Add the watercress and toss for 1 minute until just wilted. Season lightly with salt.

Arrange a bed of wilted watercress on each plate, set a grilled portobello on top gill-side up, spoon béarnaise generously into the cap, and finish with a pinch of flaky sea salt.