n a small bowl create a marinade for the burgers by combining the garlic, cilantro, Thai basil, ginger, lemongrass, fish sauce, soy sauce, brown sugar and pepper. Add the pork, beef, and shiitake mushrooms to the bowl. Using a wooden spoon or spatula, mix until the marinade is completely incorporated into the meat mixture. Place plastic wrap, or other seal, tightly over the bowl and place the burger mixture into the refrigerator for 30 minutes to 2 hours.
Meanwhile, quick pickle your vegetables by combining the water, unseasoned rice vinegar, fish sauce, minced ginger and salt into another medium sized bowl. Whisk to combine. Add in shredded carrots and beets. Cover tightly with plastic wrap, or other seal, and refrigerate until you are assembling the burgers.
While your burgers are marinating and your vegetables are pickling, mix the mayonnaise, Sriracha and lime juice together to create your spicy lime mayo. This, too, can be covered tightly and refrigerated until you are ready to assemble.
To prep your burger toppings, heat a cast iron pan over medium to high heat. Add 1 tablespoon of sesame oil. Once hot and glistening, add each oyster mushroom top down into the pan. Drizzle with 1 tablespoon of soy sauce and season with pepper. Using another heavy skillet, press down on the mushrooms to flatten. Leave the skillet on the mushrooms and cook, undisturbed until they begin to slightly crisp and turn golden brown, around 4 minutes. Remove the skillet on top, add 1 teaspoon of fish sauce, swirling around the cast iron. Flip over the mushrooms and continue to cook around 3-4 more minutes or until the underside is also golden brown. Remove the oyster mushrooms from the cast iron and cool on the side. Keep the heat on on the skillet.
Take the burger meat out of the refrigerator and, using your hands, form two equally sized patties. The patties may seem slightly wetter than usual, but this is perfectly fine. Place the patties into the cast iron skillet and cook undisturbed on medium heat for around 6 minutes or until a good crust has formed and the meat easily releases itself from the skillet. Flip over the burgers and cook for an additional 4-5 minutes or until cooked through. Remove the burgers from the cast iron and rest them on a cutting board or plate.
Time for assembly! Cut a kaiser roll in half and lightly toast it. Spread spicy lime mayo on the bottom half of the bun. Top with a burger. Remove the pickled vegetables from the refrigerator and, using a fork, lift out some of the carrots and beets. Press the vegetables into the side of the bowl to get rid of excess pickling liquid. Place some pickled carrots and beets on top of the burger patty. Top this with 2 of the cooked oyster mushrooms. Finally, sprinkle with chopped cilantro, chiffonade Thai Basil and thinly sliced spring onion. Place the top of the bun on the burger and serve with 2 lime wedges.
Best enjoyed with my smashed air-fryer potatoes with reserved spicy lime mayo!