Heat the oil in a frying pan and gently cook the onion for about 5 minutes until soft. Set aside.
Mix together all the ingredients in a bowl, add the cooked onion and season to taste. Add some crushed chilli flakes or chopped fresh chilli if you like the burgers to have more heat.
Shape the mixture into 6-8 burgers and chill for 10-15 minutes.
Fry, grill or barbecue the burgers for about 5 minutes each side, depending on how well done you like your burger.
Cut the buns in half and toast or barbecue them cut-side facing down.
Put a lettuce leaf on the lower part, add the burger, topped with a few slices of gherkin & spring onion, then drizzled with sweet chilli sauce. Cover with the upper half of the bun and enjoy!