If making more burgers with this mixture, reduce the cooking time for each side. You want a golden brown crust on each side.
Adjust the soy sauce or Worcester sauce levels to your liking, this is the standard recipe but I have lessened both of these slightly when I make it each time.
Why not cook off some chilli with the onion or add chilli powder or other herbs to suit the meat you are using.
You can try these in the oven with an ovenproof dish, 200 C / Gas 6 and 20-25 minutes should do it, turning once.