Bring the ground beef to room temperature about 20 to 30 minutes prior to cooking. Cut the onion into slices about a quarter inch thick, but don’t separate them into individual rings.
In a small bowl, whisk together the ingredients for the sauce and chill until ready to serve.
Preheat a skillet or flat-top griddle over high heat. Place the onions onto the skillet and cook for 2-3 minutes.
Meanwhile, roll the ground beef into four equal-sized balls. Place onto each slice of onion on the grill and use a spatula or burger press to smash the meat onto the onion so that the onion is completely covered with the meat and the meat is fully touching the skillet on all sides of the onion.
Sear for 4-5 minutes per side for well done. Season both sides with salt and pepper, to taste.
Remove the burgers from the skillet and rest for 5-10 minutes for best results. Serve with sweet burger buns, lettuce, pickles and the sauce to top it off. Enjoy!