Place a medium skillet over medium-high heat with about 1 tablespoon olive oil.
Add the onion to the pan and cook until tender, 4-5 minutes.
Remove to a large mixing bowl and wipe the pan clean.
When the onions have cooled slightly, place the pan back over medium-high heat with about 2 tablespoons olive oil.
To the mixing bowl with the onions, add the sirloin, Worcestershire, some salt and pepper and mix thoroughly to combine.
Divide the meat into 4 portions and shape into 4 thin burgers.
Add the patties to the hot pan and cook until golden brown and cooked through, about 4 minutes per side.
While the burgers are cooking, butter up one side of each slice of bread then flip them over and spread mustard on each.
When the burgers have finished cooking, make stacks by laying down the slices of white bread, butter side-down, and top them with a slice of cheese, a patty, another slice of cheese, 2 slices of bacon and tomato and another slice of bread (butter side-up).
Wipe out the pan that the burgers were cooked in and return it over medium heat.
Add the stacks to the pan and cook until the bread is toasted golden brown and the cheese has melted, 2-3 minutes per side.