Place all of the burger ingredients in a medium bowl and mix with your hands or a fork until evenly combined.
To check the seasoning, make a tiny little patty of meat and quickly fry it until it is cooked. Taste and adjust the seasoning accordingly.
Shape into up to six, evenly sized burgers. Put in the fridge to firm up.
Meanwhile, make the relish. Heat the oil in a small saucepan. Add the onion and ginger and cook gently until the onion is soft – around eight minutes. Add the plums, sugar and 50 ml (2 fl oz) cold water. Cook until the plums have softened, about ten minutes. Add the vinegar and orange zest, and cook for a further minute. Season to taste. You can keep this in a jar in the fridge for up to two weeks.
Grill or fry the burgers, depending on your preference, then serve with the relish, in burger buns of your choice.