Preheat the oven to 425°F. On the stovetop, cook the beef in a large skillet over medium heat until it's no longer pink, three to four minutes. Break the cooked beef into crumbles, and drain the fat off it.
Place the pizza crust on an ungreased baking sheet or pizza pan. Mix the ketchup and mustard together, then spread over the crust. Top evenly with the crumbled beef. Bake the pizza for five minutes, then take it out of the oven.
Sprinkle the pizza with cheese, and bake it for 8 to 10 minutes longer. It's done when the cheese is bubbly and the crust golden brown.
Top the pizza with the lettuce, pickle and onion. Then whisk the mayonnaise and pickle juice together, and drizzle over the pizza.
