• Preheat the oven to 180°C.
• To make the Big Zac secret sauce, combine the ingredients in a bowl and set aside. Place the dried shiitake mushrooms in a food processor and blitz to a powder. Add the black beans and process until smooth.
• In a large bowl, combine the TVP, reserved aquafaba, ketchup, barbecue sauce and liquid smoke. Mix in the black bean mixture and set aside. In another large bowl, combine the gluten flour, stock powder, onion powder and pepper. Stir the prepared TVP mixture into the dry ingredients until the mixture comes together.
• Knead until everything is fully combined, adding the extra gluten flour if needed to bring it together into a ball. Divide into quarters and press each quarter into two patties.
• Line a baking tray with baking paper and place the patties on top. Sprinkle over 125 ml (½ cup) of water.
• Cover the patties with another sheet of baking paper and bake for 1 hour, flipping at the halfway mark and sprinkling over another ½ cup water. Check every so often to ensure that the patties aren’t browning up too much – they should be mostly dry by the time the hour is up.
• Remove from the oven and set aside to cool.
• Heat half the oil in a frying pan over medium heat. Add four of the patties and press with a spatula to flatten them slightly. Cook for 5 minutes, then flip and cook the other sides for 5 minutes. Flip again and place two cheese slices on top of each patty. Continue to cook until the cheese starts to melt, then remove from the pan.
• Heat the remaining oil and cook the remaining patties.
• While the patties are cooking, use a bread knife to halve the buns. Four of the top halves won’t be required, so set these away for later use. Lightly toast the buns.
• Spread a generous amount of Big Zac secret sauce over the bottom half of four burger buns. Scatter lettuce over the sauce and top with a cheesy burger patty, cheese side down. Add a little onion and place the remaining bottom bun halves on top.
• Continue to build the burgers with another round of sauce and lettuce and then top with the pickle and remaining patties. Scatter over the remaining onion and finish with the bun tops.

Mayonnaise (optional)

• Place all the ingredients except the oil in the bowl of a stick blender or plastic jug. Blend with a stick blender for 20 seconds or so, until frothy.
• With the stick blender running, slowly pour in the oil in a steady stream for 1–2 minutes, until you have an emulsified and thick mayonnaise. Taste, and adjust the seasoning if necessary.
• The mayonnaise will keep in an airtight container in the fridge for up to 4 weeks.