Chop one of the onions into fine dice and slice the other into 0.5cm thick rings, set the rings aside. Mix the diced onion, mince, egg, breadcrumbs, cumin seed and dijon mustard together. Season with salt and black pepper. Divide the mince mixture into 4 and form into burgers, place in the fridge for 30 minutes to firm up. Meanwhile heat 1 tbsp of the oil in a frying pan and lightly fry the onion rings for a couple of minutes until they start to brown and soften. Remove from heat and set aside. Heat a grill, griddle or barbecue until hot. Rub the burgers with the remaining oil and cook, rotating after 3 minutes, for roughly 6 minutes per a side, until burger is cooked through but still juicy. Remove and leave to rest. Toast the cut side of the hamburger bun and top each bottom piece with lettuce and a burger, then layer on the sliced cheese, onions rings and slices of tomato.
