Vegan Grilled Portobello with Béarnaise Sauce

Scrape the gills from the portobello caps with a spoon and place caps gill-side up in a shallow dish. Whisk together the tamari, olive oil, balsamic vinegar, garlic, thyme, and black pepper. Pour over the mushrooms, turning to coat. Mince the shallots, chop the tarragon stems, and measure all remaining ingredients.

Cover the marinating mushrooms and set aside at room temperature for at least 30 minutes, turning once halfway through.

Combine the white wine, white wine vinegar, minced shallots, tarragon stems, and peppercorns in a small saucepan over medium heat. Bring to a simmer and reduce until only about 2 tablespoons of liquid remain and the mixture looks syrupy — approximately 8–10 minutes. Strain through a fine-mesh sieve, pressing the solids, and allow the liquid to cool completely.

Place the chilled aquafaba, Dijon mustard, and 1 tsp white wine vinegar in a heatproof bowl set over a saucepan of barely simmering water (bain-marie), ensuring the bowl does not touch the water. Whisk vigorously for 3–4 minutes until the aquafaba thickens slightly and turns pale and foamy — keep the temperature gentle throughout, well short of a boil.

Remove the bowl from the heat and, whisking constantly, begin adding the warm melted coconut oil in an extremely thin, steady stream — drop by drop at first, then increasing to a thin trickle — until all the oil is incorporated and the sauce is thick, glossy, and coats the back of a spoon.

Whisk in the cooled béarnaise réduction, the chopped tarragon, parsley, turmeric, salt, and white pepper. Taste and adjust seasoning. Keep warm by setting the bowl back over the hot (off-heat) water, whisking occasionally.

Heat a cast-iron grill pan or outdoor grill to its highest setting until visibly smoking. Lift the portobellos from the marinade, shaking off excess, and place gill-side down on the grill.

Grill without moving for 4–5 minutes until deep char marks form and the mushroom releases easily from the grate.

Flip the caps and grill for a further 3–4 minutes until the gill side is caramelised and the mushroom has collapsed and softened throughout.

Transfer the portobellos to a warm plate, season with flaky sea salt, and rest for 2 minutes. Spoon the béarnaise generously over each cap and serve immediately alongside watercress and lemon wedges.