!To Make the Sauce:
Mix all the sauce ingredients together in a small bowl and set aside (or cover and refrigerate until ready to use).
!To Make the Onions:
Heat oil in a frying pan over medium heat. Add onions, coat with oil and salt generously. Cook, stirring and tossing often, adding small amounts of water (1T at a time) whenever onions appear to be dried out (you'll need to add water several times). Cook 10-15 minutes until onions are very soft and browned. Set aside.
!To Make the Burgers:
Heat a large frying pan over medium heat. Toast the interior of the buns. Place the toasted buns into a 200 degree oven to keep warm.
Season the patties with salt and pepper. Place into hot pan. While they cook on the first side, spread 1T of mustard onto the raw sides of the patties. Flip. Place a cheese slice on each patty. Remove patties from pan when cheese is well melted and burgers are cooked through.
!To Assemble Burgers:
On the bottom bun, spread a generous amount of the sauce. Top with 4 pickle chips followed by tomato and lettuce, then patty. Place the grilled onions directly on top of the cheese. Complete with top bun and serve.