Saute the diced onion in the beef tallow for 3-4 minutes, then set aside. Wipe the remaining tallow out of the pan and set it to high heat.
Cook 2 smashburger patties, season with Lawry's, top each with American cheese and set aside.
Add the onions back in along with the beef stock to deglaze and lift all the flavour off the bottom of the pan. Add the burgers back in with the bacon jam and diced pickles and roughly chop everything together.
Add the Worcestershire, yellow mustard, a little more bacon jam and 2 more slices of cheese, and chop again until well combined.
Place the hoagie roll on top to steam, then scoop the chopped cheese into the roll. Add burger sauce if you like, and wrap the sandwich so it steams a little before serving.
By @alfskitchen_. Don't skip wrapping the sandwich so it steams.