Heat the oil in a frying pan, add the shallots and lower the heat. Let them soften for aprox. 8-10 minutes. Add the garlic and cook for 2 minutes.
Once softened mix together in a large bowl with the beef, thyme, parsley, gherkins, Dijon mustard, sweet chilli sauce and season well.
Crack the egg into a small bowl, whisk and add to the beef mixture. Mix it through but don't be tempted to overwork it or the burgers will be tough.
Form into four even patties. Transfer to a bowl or plate and refrigerate over night or minimum, an hour.
Cook the burgers over a hot griddle, under the grill or on the barbecue until done to your liking. 5 minutes either side is a good guide, maybe a little less on the BBQ.
In the last couple of minutes top with a thick slice of cheddar. If using halloumi fry the halloumi until golden and add to the top of the burger when serving.
Serve in the brioche buns with a handful of rocket and a big dollop of tomato chilli jam, some lettuce and sliced tomato.