Start by mixing all of the Vegan Big Mac Sauce ingredients together in a mixing bowl.If you purchased thick vegan burger patties, slice them in half length ways to create two thinner patties. If the patties are reasonably thin already, skip this step.Then place the vegan burgers in the oven, and follow the instructions on the packet to ensure you cook them well.Dice the onion into small pieces.Slice the gherkins into thin circles. Slice the lettuce then shred into thin strips.Now take your burger buns and very carefully cut them into three pieces. If the buns are too thin to slice into three. Use another bottom half of a bun as the middle bun.Spread the Vegan Big Mac sauce you prepared earlier onto the sliced bread rolls, top and bottom.Start building your burger by arranging the lettuce, onions then gherkins on the bun in that order.To give the cheese time to melt, place them on to the burger patties in the oven two minutes before the burger patties should come out of the oven.Take the patties out of the oven and complete your burgers.Enjoy!If you fancy cooking up a meat version of this delicious recipe, take a look at our copycat Big Mac Recipe.