Preheat a large char-grill pan or barbecue over medium-high heat. Rub beef patties with oil and season. Cook patties for 2-3 minutes or until bottoms are lightly charred. Turn patties and top with cheese. Cook for a further 2 minutes, or until bottoms are lightly charred and cheese has melted. Set patties aside on plate loosely covered with foil.
Spread mayonnaise over bottom halves of buns. Top with baby cos, beef patties, tomato, pickles and burger patties. Spread ketchup on top half of buns and place on top of each pattie, to serve.
Extra burger patties can be frozen, with baking paper in between each pattie, and thawed in the fridge.
Use your choice of condiments (mustards, relishes, chutneys all work well here too); swap baby cos lettuce for iceberg or mixed leaf; swap tasty cheese for Colby, Gouda or American Jack; add beetroot, red onion, pineapple or a fried egg for an Aussie Burger.
Serve with French fries or sweet potato fries, if desired.