1. Place the pork mince in a large bowl. Add the onion, mustard, Stilton and sage and mix well together. Shape firmly into four even-sized burgers.
2. Place the burgers in the fridge for 30 minutes to rest before cooking.
3. Preheat a ridged griddle pan or frying pan and cook the burgers for 4-5 minutes on each side until browned and completely cooked through.
4. Meanwhile, split the buns in half and toast the cut sides.
5. Serve the burgers with the lettuce leaves in the toasted burger buns, garnished with tomato and gherkin.