Preheat oven to 425 degrees F. Slice bun tops into 1/4-inch strips (if bun tops are very thick, slice bottoms off horizontally before slicing into strips). Arrange in a single layer on Large Bar Pan; bake 8-10 minutes or until lightly toasted. Remove to Stackable Cooling Rack; cool completely.
Slice half of the onion crosswise into thin rings, using Ultimate Mandoline filled with thin slicing blade. Cut tomatoes into quarters lengthwise and slice crosswise using Utility Knife. Set onion and tomatoes aside.
Cook ground beef in (8-in.) Saute Pan over medium-high heat 5-7 minutes or until no longer pink, breaking beef into crumbles using Slotted Turner. Chop remaining onion half using Food Chopper. Finely dice pickles using Utility Knife. In Classic Batter Bowl, combine chopped onion, pickles, ketchup and mustard. Add cooked ground beef; mix well using Small Mis 'N Scraper.
To serve, arrange lettuce on large serving platter. Spoon beef mixture over lettuce. Top with cheese, tomatoes, and sliced onion. Arrange hamburger bun croutons around edge of platter. Serve immediately.