1. Preheat the barbecue and preheat the oven to gas 6, 200°C, fan 180°C. Put the mushrooms on a baking tray and brush with the oil; season. Bake for 10 mins or until starting to soften.
2. Meanwhile, whisk together the marinade ingredients in a small bowl. Remove the mushrooms from the oven and pour over the marinade; set aside to marinate for 10 mins or up to 24 hrs ahead.
3. Transfer the mushrooms with a slotted spoon to the barbecue (discard the marinade) and cook for 3-4 mins each side. Press down a few times with a fish slice until you have dark charring marks and the mushrooms are starting to crisp up.
4. Add a slice of cheese-alternative to the mushrooms, stalk-side up, and grill for a further 1 min until starting to melt. Lightly brush the cut sides of the buns with the melted spread, then toast on the barbecue until golden and charred.
5. To assemble, spread the mayo on each bun and top with lettuce, tomato, a mushroom and onions. Top with the other half of the bun to serve.
Tip: If you don’t have a barbecue or prefer to cook the mushrooms on the hob, start by heating a heavy-based griddle pan over a medium-high heat. Add the mushrooms, stalk-side down then put a smaller, heavy-based pan on top to flatten them. Griddle for 2-3 mins, flip, press down again and cook for a further 2-3 mins until charred. Continue the recipe from step 4 to finish.
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