Place a pizza stone or heavy baking sheet in the oven. Preheat the oven to 400 degrees F.
While the oven preheats, heat a nonstick skillet over medium heat. Add the beef and onion, and sauté until the beef is browned and the onion is tender. Crumble the meat and sprinkle with the steak seasoning as the mixture cooks. Remove the pan from heat.
For the sauce: combine the mayonnaise and ketchup in a small bowl, stirring to fully blend. Spread the mixture evenly over the pita rounds, leaving a small border. Distribute the beef mixture evenly over the sauce on each pita, and then evenly sprinkle the cheeses over the beef. If using, top with the tomato slices and pickles.
Carefully transfer to the baking sheet or stone and bake for 8 minutes, give or take a minute or two, or until the pizzas are hot throughout and the cheese is completely melted. Remove from the oven, and sprinkle with lettuce, if using. Cut the pizzas into 4 wedges each and enjoy.