Preheat oven broiler with rack in top third of oven. Line a sheet pan with foil for easy cleanup. If you're grilling the burgers, preheat and grease grill.
Grate the onion over a large bowl, catching all the juices, or finely mince the onion if you prefer.
Add the rest of the ingredients, mixing well. (Use your hands but don't over-mix).
Shape into patties about ½ inch thick (make them slightly thinner in the middle). If grilling, spray each side lightly with nonstick spray.
Broil on high for 6-7 minutes, flip, and continue broiling 4-6 more minutes or until burgers are done. Internal temperature should be 165°F when instant read thermometer is inserted into the center of the burger. If you're grilling the burgers, don't flip the burgers until they are firm and release easily. They tend to break apart easily.
Broiling/grilling time will vary depending on how thick the burgers are made and how hot your grill is.
Make-Ahead Idea: Patties can be made a day ahead; refrigerate or freeze them. They'll be ready to cook up when you're ready to eat. Or cook them up and freeze them.
Raw burgers: Cook as directed. Frozen raw burgers: Cook as directed, adding a few minutes to the cooking time.
Cooked burgers: Reheat in the microwave or in a skillet on the stove. Frozen cooked burgers: For best results, thaw overnight in the fridge. Reheat in the microwave or in a skillet until heated through.