The ultimate BBQ burger

Method

1. For the beef burgers, remove the bone marrow from the bone using a teaspoon, then finely dice it into 3mm cubes. Gently mix it through the mince, then roll into 2 equal-size balls and flatten. Chill in the fridge, covered (see Make Ahead).
2. For the burger sauce, put the egg yolk, mustard, lemon juice and vinegar in a bowl and whisk until combined. Slowly pour the oil into the egg mixture while whisking, so it thickens like a mayonnaise – doing this slowly prevents the sauce splitting (you could use a blender or food processor instead.) When the sauce has thickened, add the ketchup and season with salt and pepper to taste. Transfer to a sealed container and store in the fridge until ready to use.
3. Before cooking, bring the beef burgers up to room temperature (see Make Ahead). If not cooking on a barbecue, heat a large griddle pan over a high heat and brush with oil. Season the burgers generously with salt and pepper, then put them on the barbecue/griddle pan. Cook for 5-10 minutes on each side until browned and cooked through. Remove the burgers from the heat to rest for a few minutes while you prepare the remaining ingredients.
4. Split the burger buns and toast the insides on the barbecue/griddle pan for 2 minutes or so until golden.
5. Layer the lettuce and pickles onto the buns, add the burger, cheese and burger sauce, then top with the tomato and the bun tops.