Preheat oven to 425°. In a large skillet, cook beef over medium heat until no longer pink, 3-4 minutes, breaking it into crumbles; drain., Meanwhile, place crust on an ungreased baking sheet or pizza pan. Mix ketchup and mustard; spread over crust. Add ground beef; bake 5 minutes. Sprinkle with cheese; bake until cheese is bubbly and crust is lightly browned, 8-10 minutes longer., Top with lettuce, pickle and onion. Whisk mayonnaise and enough pickle juice to reach desired consistency; drizzle over pizza.