Bake your FatHead pizza crust according to the directions in the FatHead Pizza recipe.
In a bowl mix together your Big Mac sauce. Spread about 3/4 of the sauce over the FatHead pizza crust. Then add the cooked ground beef, red onions, and cheese. Bake in the oven at 220C/425F until the cheese is melted.
Remove the pizza from the oven and add the lettuce, sliced tomatoes, and pickles. Drizzle the remaining sauce over the top of the pizza. Slice and serve.