Place the soy, honey, ginger and lemongrass in a large bowl and whisk to combine. Cut the mushrooms into 2.5cm (1 inch) slices and place in the bowl with the marinade and toss well to combine. Refrigerate for at least 1 hour.
While the mushrooms are marinating make the pickled carrots. Place the rice wine vinegar and sugar in a saucepan and place over a medium low heat. Stir until the sugar is dissolved. Leave to cool for a few minutes before pouring over the carrots.
To make the sriracha mayonnaise combine the mayonnaise and the srirache and whisk to combine.
Place a large frying pan over a medium high heat. Add half the olive oil and half the mushrooms and cook for 2 to 3 minutes before flipping the mushrooms over and cook for a further 2 minutes. Repeat with the remaining mushrooms.
To assemble the banh mi slather the baguettes with the srirache mayonnaise and top with the mushrooms, cucumber, pickled carrots and coriander (cilantro) leaves.
If you don't have sriracha you can use your favourite chili or sweet chili sauce.