Heat the oil in a non-stick pan over a medium heat and add the sausage burgers. Cook for 3 minutes on each side, then remove from the pan. Add the cheese slices to the cooked burgers and allow them to melt over the burger.
Lay the burger buns face side up on a baking tray and toast in the oven for 4 to 6 minutes. Add 1 tablespoon of onion relish to the base of each burger bun, then add some salad leaves and top with the cheesy burgers. Add a little more onion relish, then top with slices of onion and tomato and the remaining burger bun. Serve immediately.