Preheat oven to 425°F Slice bun tops into ¼-in. strips (if bun tops are very thick, slice bottoms off horizontally before slicing into strips). Arrange in a single layer on large 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. Remove and discard stems from tomatoes; cut tomatoes into quarters lengthwise and slice crosswise. Set onion and tomatoes aside.
Cook ground beef in (8-in.) Sauté Pan over medium-high heat 5-7 minutes or until no longer pink, breaking beef into crumbles.
Chop remaining onion half using Food Chopper. Finely dice pickles.
In Bowl, combine chopped onion, pickles, ketchup and mustard. Add cooked ground beef; mix well.
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.