Start by preparing the special sauce: in a small bowl, whisk together the vegan mayonnaise, ketchup, mustard, and diced onion until smooth and well combined. Set aside to let flavors meld.
Toast the sesame seed buns in the oven or toaster until lightly golden, about 3-4 minutes, and set aside.
Heat a skillet over medium heat and cook each veggie burger patty for about 3-4 minutes per side until browned and heated through, then remove and set aside.
Spread a generous layer of the prepared sauce on the bottom bun, then layer with shredded vegan cheese, lettuce leaf, and chopped pickles. Place the cooked veggie patty on top of the pickles.
On the top bun, spread more of the special sauce or leave plain as desired. Place the top bun on the assembled layers.
Optionally, sprinkle toasted sesame seeds over the top bun for added texture and flavor. Slice the sandwich in half if desired.
Serve the Veggie Big Mac immediately, accompanied by your favorite side. Enjoy the layers of crunchy, creamy, and savory flavors!