For those of you who can't make it out to the West Coast.
2 leaves fresh iceberg lettuce, white core section removed, torn to bun-sized piecesCombine all ingredients in a small bowl, and stir to combine. Cover and refrigerate until ready to use .Place feed shaft, blade, and 1/4-inch die of meat grinder in freezer until well-chilled. Meanwhile, place meat chunks on rimmed baking sheet, leaving space between each piece and place in freezer for 10 minutes until meat is firm. Combine meat in large bowl and toss to combine.
Once onions begin to sizzle heavily and appear dry, add 1 tablespoon water to skillet and stir to scrape up any browned bits. Continue cooking until water evaporates and onions start sizzling again, about 1 minute. Repeat process, adding 1 tablespoon of water at a time, until onions are meltingly soft and dark brown, about 3 times total. Transfer to a small bowl and set aside.Place closed buns in preheated oven until slightly darkened and crisped, about 2 minutes.