Preheat the oven to 220°C (450°F) or 200°C (fan-forced).

In a microwave-safe jug, melt the butter by microwaving it for about 30 seconds.

Stir in the minced garlic, salt, and pepper until well combined.

Brush the smooth side (top) of each mushroom cap with the garlic butter.

Place the mushrooms on a baking tray, gills side up (stems facing upwards).

Brush the remaining garlic butter onto the gills and around the edges, ensuring you use all the garlic.

Drizzle a little olive oil over the mushrooms for extra richness.

Roast the mushrooms in the preheated oven for 25 minutes.

After 25 minutes, flip the mushrooms and brush them with the juices collected on the tray.

Roast for another 10 minutes until the center of the mushrooms is tender.

While the mushrooms roast, heat olive oil in a large skillet over high heat.

Add the sliced onion and cook for 5 to 7 minutes, stirring occasionally, until golden and soft.

Season with a pinch of salt and pepper.

In a small bowl, mix together mayonnaise, ketchup, Dijon mustard, lemon juice, and Sriracha (if using). Adjust to taste.

Once the mushrooms are tender, place a slice of cheese on each mushroom and return to the oven for 3 minutes, just until the cheese melts.

At the same time, place the burger buns in the oven to lightly toast.

Spread a dollop of Pink Sauce on the base of each toasted bun.

Add a lettuce leaf, a slice of tomato, and then place the garlic butter mushroom on top.

Pile on the caramelized onions.

Spread a little more Pink Sauce on the top bun and place it on the burger.

Serve your garlic butter mushroom burgers immediately, while the mushrooms are warm and juicy. Enjoy with your favorite sides!