Mix the onion, mince, mustard, egg and some seasoning together, then use your hands to form into 4 burgers. Cover and chill in the fridge for 10 minutes.
Heat a large non-stick frying pan, then brush the burgers with oil and cook for 3-4 minutes on one side, then flip them over and cook for a further 3 minutes, topping with the Red Leicester and Sage Derby Cheese for the final minute.
Once the Cheese has melted, transfer the burgers to board and lightly toast the inside of the burger buns in the frying pan, letting them soak up any juices from the burgers.
To serve, put some lettuce on the base of each bun, then top with the burgers, sliced tomatoes and any condiments.