Preheat the oven to 390F. Heat a small frying pan over a medium heat and fry the onions until softened.
Combine all the ingredients into the bowl of your food processor and mix until completely combined. Be careful not to over-mix - you don't want gloopy burgers, you want thick, chunky burgers packed with texture. Form the mixture into 6 burgers.
Lay the burgers on a baking tray and bake for around 30 minutes, or until crispy and golden on the outside.