Place the mushrooms in a bowl and pour the buttermilk in to coat. Let it sit for a couple of minutes.
Preheat your oven to 200c (fan forced).
In a medium sized bowl using a whisk, mix together the flour, Panko breadcrumbs, baking powder and herbs and spices. Set aside.
Place the baking dish in the oven so it gets hot (around 10 minutes). While the dish is heating up, remove the mushrooms from the buttermilk, shaking off any excess, and dust in the seasoned flour mixture, coating completely, covering well.
Remove the baking dish from the oven, then add the melted butter so it coats the base of the dish.
Arrange the mushrooms on top, without touching the sides.
Bake for 10 minutes, uncovered. Remove from the oven then flip the mushrooms over and bake for another 10 minutes.
Serve on a soft bread roll of your choice (rye, brioche, soft white, sour dough or multigrain), with cheese, lettuce, tomato slices and kewpie mayo, or any of your favourite burger toppings or sauces.