For the buns, whisk the yeast and sugar into the water and milk and leave to sit while you get on with the next bit.
Add a teaspoon of sea salt to the flour and rub in the butter with your fingertips until it resembles breadcrumbs. Stir in one of the eggs and the yeasty mix.
Tip the mix on to a generously floured surface. The dough will be super sticky but bear with it and knead for 10 minutes. Add to an oiled bowl and leave to rise for 2 hours.
After two hours, knock it back then reform into your bun shapes. Cover loosely and allow to prove for a further hour. Preheat the oven to 200C.
Beat the other egg and glaze the top of the rolls. Bake for 20 minutes or until puffed up and brown, and cool on a wire rack before using.
For the burgers season the beef generously with salt and pepper, then form into patties (you should get 2 – 3). Allow to rest in the fridge for 30 minutes before using, this cools the fat down to keep them together while cooking.
For the burger sauce, mix the ingredients together and keep refrigerated until needed.
Lightly fry the onion slices in olive oil until softened and keep aside until needed.
Get a non-stick frying pan over a medium-high heat. Cook the patties, flipping every minute until cooked to your liking. For the last minute of cooking brush the top side with oyster sauce for a beautiful shine and delicious savouriness.
Assemble your burger with tomato, lettuce, gherkins, onions and burger sauce in your bun. Devour messily with a dining companion.