Place the bread crumbs in a large bowl and add buttermilk, let stand for 5-8 minutes to soak (this will help bind the mixture).
While the bread is soaking, heat a frying pan, add olive oil and sauté the diced onion with 1 heaped teaspoon of Melbourne Coffee Meat Rub. Cook until onions are soft and transparent.
Add the onions, beef mince, remaining Melbourne Coffee Meat Rub, lemon zest, parsley and salt to the bread crumbs and mix well.
Divide the burger mixture into 6 portions shaping into round patty shapes with your hands; place on a tray with baking paper; rest in the fridge for at least 30 minutes to firm up. These can be made the night before.
Heat grill or pan to medium - high heat. Lightly coat the burger patty with olive oil and place it on the grill or pan. Cook for 7-8 minutes to create a crust on one side then turn over. Place a slice of vintage tasty cheese on the patty to melt. Cook for a further 7-8 minutes (check internal temp: medium-rare: 130-135°C; medium-well: 150- 155°C).
When ready to build the burger, place the inside of the buns face down on the BBQ grill to toast, spread 1 tsp of mustard on each bun, top with rocket, sliced tomato, red onion, beef patty, sliced cucumber pickle. Top with lid and serve with a side of onion rings.