Heat the butter in a small pan and gently cook the onions over moderate heat until completely softened. Add the vinegar to deglaze the pan and reduce the heat to very low to continue to cook them until jammy and all the vinegar has evaporated. Set aside to keep warm.

In the meantime, preheat the oven to 180°C (fan oven). This is 200°C for conventional ovens (or 400°F / gas mark 6).

Wipe the mushrooms clean and gently remove and discard the stalk without damaging the mushroom.

Place the mushrooms on a roasting tray and brush with a very small amount of truffle-infused olive oil. Place in the oven for 7 minutes, then remove the tray from the oven. Using a spatula and some kitchen paper, tip each mushroom at an angle to drain any liquid.

Divide the sliced Brie between the ‘shrooms and return the tray to the oven for 5 minutes, or until you can see the cheese has melted into a gooey mess.

Remove the grilled brie baked mushrooms from the tray using a spatula (I advise tipping it slightly again to allow excess moisture to run off) and serve with the onion chutney. Garnish with picked thyme leaves and finish with small scatter of Pink Himalayan Salt.