Wrap the crackers in a tea towel and smash up until fine, breaking up any big bits with your hands, then tip into a large bowl. Finely chop the parsley (stalks and all), then add to the bowl with the mustard (if using), and the minced beef. Crack the egg into the bowl, then add a good pinch of sea salt and black pepper. With clean hands, scrunch and mix everything up well. Divide into 6 and pat and mould each piece into a roundish shape, roughly 2cm thick. Place the burgers onto a plate, drizzle with oil, then cover and place in the fridge until needed – this will help to firm them up. To cook the burgers, preheat a large griddle or frying pan for 4 minutes on a high heat, then turn the heat down to medium. Place the burgers on the griddle or into the pan, then use a spatula to lightly press them down, making sure the burger is in full contact with the pan. Cook the burgers for 3 or 4 minutes on each side, depending on how you like them – you may need to work in batches. Meanwhile, wash and dry a few small lettuce leaves, tearing up the larger ones. Slice the tomatoes, peel and finely slice the red onion, then slice the gherkins lengthways as finely as you can. Place all the veg toppings, and the cheese (if using) on a platter and put in the middle of the table with plates, cutlery, ketchup and drinks. Once cooked, remove the burgers to a plate and carefully wipe the pan clean with a ball of kitchen paper. Halve the burger buns and lightly toast them in the pan, then serve up.