Heat the oil in a frying pan over a medium heat and fry the onion for 7-8 minutes until soft, adding the garlic and mustard powder for the final minute.
Add the sweetcorn and chilli and cook for a further 2 minutes. Add the vinegar and agave nectar and stir together. Turn the heat up to high and simmer for 2 more minutes. Allow to cool, then season and stir in the coriander.
Put the vinegar, sugar and salt into a small saucepan and heat over a medium-high heat until steaming, making sure the sugar and salt have dissolved. Remove from the heat. Put the sliced onion in a small bowl and pour over the hot liquid. Allow to cool to room temperature for 15 minutes, then drain and toss with the dill.
Cook the burgers according to the instructions on the packet. Mix the yogurt, lemon juice and cumin together in a small bowl. Spread a little yogurt over the bottom of each bun and top with two pieces of lettuce and the burger on top. Drizzle a little more yogurt over the burger, then top with a slice of tomato and a dollop of relish or pickled onion. Top with the bun and serve immediately with a handful of salad.