To make the pickled onions, place all the ingredients in a pan and bring to the boil. Take off the heat and allow to cool.
In a saucepan, soften the onions and garlic in vegetable oil on a medium heat. Then add the spices, ketchup and vinegar. Drain the liquid from the tinned jackfruit and add the jackfruit to the pan. Slowly simmer for 20 minutes, gradually pulling the jackfruit apart with a fork or tongs. Finish with the marmite and soy sauce. Set aside.
Slice the burger buns in half and lightly char them on a grill. Fill with the jackfruit, pickled onions, cucumber, cabbage and mayonnaise.