Form ground beef into 8 patties, 1/4lb each, salt and pepper both sides of patties.
Bring flat grill or heavy frying pan to medium-high heat.
Have a flat weighty ready to “smash” the burger patties. I used a saucepan.
Add 1 tbsp vegetable oil or bacon fat, and place patties down to cook.
Wait one minute and press the patties down to flatten, continue cooking for 2-3 minutes, then flip.
After about one minute, smash the second side, and continue cooking until desired doneness.
Add the cheese slice about one minute before removing the patties from the grill.
Set patties aside, turn the heat down to medium, then toast the buns in the hamburger drippings.
Slather mustard on both buns, place lettuce down first, then the double stack of hamburger patties.
Top with slices of onion, and then a big scoop of Bacon-Onion Jam, and dig in!