MAKE THE BURGERS
If you have a meat grinder, grind your beef and bone marrow. Alternatively, ask you butcher to do this. Form the mince mix into 8 equal balls.
SET UP YOUR EGG
Set up your EGG for direct cooking, with the Stainless Steel Grid and a Cast Iron Skillet on top. Your target temperature is 200˚C.
COOK THE STOUT ONIONS
Add the olive oil, butter and onions to a Cast Iron Skillet and cook for 25 minutes, stirring regularly until translucent, soft and beginning to caramelize. Add the stout, sugar and Worcestershire sauce, turn up the heat to 200˚C and cook until the onions are sticky and the beer has evaporated. Take off the heat and set aside.
COOK THE BURGERS
Increase the EGG's temperature to 250˚C, add a Cast Iron Plancha Griddle and get it smoking hot. Lightly toast your brioche buns, taking care not to burn them. Add the balls of mince and press them down with a spatula. Cook for 2-3 minutes, squeeze French's mustard over the top then flip.
Place a slice of cheese on each patty then close the lid of the EGG. The mustard will steam on the Plancha, ensuring your patties don't dry out and your burgers develop a lovely crust. Leave for a further 2-3 minutes, or until the cheese has melted.
Layer up your burgers with a patty, onions, another patty, more onions and a dollop of French's mustard and ketchup if you like.