In a shallow dish, whisk together the buttermilk, garlic powder, onion powder, smoked paprika, salt, and pepper. Add the mushroom caps and turn to coat evenly in the marinade. Cover and refrigerate for 30 minutes to 1 hour.

In a separate shallow bowl, combine the flour, cornstarch, smoked paprika, cayenne, mustard powder, thyme, baking powder, salt, and pepper.

Remove the mushrooms from the marinade, letting any excess drip off. Dredge each mushroom in the coating, pressing lightly so it sticks. Dip each mushroom back into the marinade, then into the coating again for a double layer.

Place the coated mushrooms on a wire rack and let them rest for 10–15 minutes to help the coating hold. While the mushrooms rest, pour enough neutral oil into a deep-sided frying pan to reach a depth of about 1 cm and heat to 175°C.

Working in batches if needed, add the mushrooms to the pan and cook for 4–5 minutes per side, until golden and crisp. Remove briefly to paper towel to drain, then transfer to a wire rack and sprinkle with flaky salt while still hot.

Slice the brioche buns in half and toast cut side down in a dry frying pan until lightly golden.

Spread ½ tablespoon of garlic mayonnaise on the top of each bun. On the bottom bun, layer caramelised onions, then pickles. Place the fried mushroom patty on top, add a slice of cheddar, and crown with the top bun. Serve immediately with your favourite burger sides.