Place a medium pan on medium/high heat and once it heats-up add the onions. Sauté for about 5’, remove from the pan and keep aside to cool.
Preheat the oven to 190°C, air circulation and elements on, rack set in the middle.
Use a large bowl where you mix all the ingredients. Form the burgers into tennis balls size, about 225g each, and place in an oven proof tray that has been lightly oiled. Press down the burgers and make a small dent in the middle as they tend to rise, and this will compensate their rise and stay flat.
Place in the oven and cook for 15’. Then increase the temperature to the max, turn on the broiler and place the rack to the top position. Cook for 4-5’ more to get a nice colouring on them.
Serve with anything you like but, in this case, I chose to serve in sesame buns, topped the burgers with Tomato/Onion sauce next to a coleslaw salad and a potato salad.