Make the Pickled Onions:
Spread the onion slices on a plate in one layer, sprinkle the salt over them and let them sit for 30 minutes.
After 30 minutes, rinse the onions in a sieve thoroughly under running cold water. Drain and dry on paper towels. Put up in a small bowl.
In another small bowl, whisk the vinegar, sweetener, and pepper together until the sweetener dissolves. Pour over the onions and chill for at least 1 hour before serving.
Make the burgers:
Divide the ground beef into 4 equal balls and mold into burger patties. Season well on all sides with the salt and pepper. Fry or grill 5-6 minutes per side until tender and cooked.
Serve with all of your toppings on the table and let everyone create their own special burger.