Before making the mushrooms, I'd recommend whipping up the aioli and slaw. For the aioli, simply blend the ingredients in a small food processor.
For the slaw, thinly slice the veggies and place in a bowl. Pour about half the aioli on the veggies and toss. Set aside until you're ready to assemble the burgers.
Split the oyster mushrooms into pieces that are about as big as the buns you'll use. Do not wash them. Instead, clean them with a napkin.
To a mixing bowl, add besan, cornstarch, smoked paprika, garlic powder, turmeric, cayenne, salt, and pepper. Start adding water. Add a little at a time until you have a thick batter.
Pour breadcrumbs on a plate. Optional: mix in ¼ tsp of red chili powder or kashmiri chili powder for color.
Coat each piece in the besan batter, followed by the breadcrumbs. Set aside until ready to fry.
In a deep pan, heat about an inch of oil over medium heat to shallow fry the mushrooms. When the oil is hot and the mushrooms sizzle when placed in it, fry them! Do it in batches if necessary. Fry until crispy and golden-brown on both sides. It should take 4-5 minutes per side.
Stir BBQ sauce, sriracha, and maple syrup in a small bowl. When the mushrooms are ready, brush them with this glaze.
To assemble the burgers, toast the buns. Add aioli to the bottom bun, followed by the slaw and BBQ mushrooms. Add some aioli to the other bun, close the burger, and shove it in your face!!!