Stir-fry the red onions in the sesame oil until tender and cooked through, about 8 mins. Set aside and keep warm.

Meanwhile, put the dark soy sauce and butter in a pan and melt together, then add the oyster sauce and stir. Brush the mixture over the mushrooms, then leave for a minute so the mushrooms can absorb the sauce. Heat the grill to high.

Grill the mushrooms until tender, about 10 mins. When ready, you should be able to slip a knife through the thickest parts quite easily.

Grill the courgette slices until they start to blister, and are brown and tender. Warm the burger buns briefly under the grill, and mix the mayonnaise with the chilli oil.

Brush the base of each burger bun with some chill mayo, then add a courgette slice, lettuce leaf and mushroom. Top with some stir-fried onions and the bun tops.