2 pounds 90% lean ground beef

1 medium white onion

1 to 2 tablespoons heavy cream, as needed

2 tablespoons unsalted butter

4 tablespoons bacon fat

Kosher salt and freshly-ground black pepper

Using a cheese grater, grate the onion with its juice into a large, wide bowl. Add the beef and one tablespoon of cream, and dash a moderate amount of salt and pepper over top of the mixture. Mix very well with your hands. Depending on how wet the mixture is, add up to one more tablespoon of heavy cream (it needs to be able to bind together). 

Divide into 6 portions, form into a ball and flatten into patties. Season each patty with salt and pepper on both sides. If refrigerating, bring the patties out of the fridge 10 minutes before cooking. 

Using a Pan:

Pan-frying is the preferred method. Bring a heavy skillet to medium-high heat. (If the pan begins to smoke, it is too hot.) Add the butter and bacon fat. Cook the patties 4 to 5 minutes per side, depending on how well-done you like your burgers. Using a spoon, periodically baste the patties with the juices in the pan. 

Using a Grill:

Heat your grill to medium-high heat. Cook the patties 4 to 5 minutes per side, depending on how well-done you like your burgers.