Place a griddle (grill) pan over a high heat.
Put the mayonnaise into a small bowl and stir through the mustard and Tabasco.
When the griddle is smoking hot, brush with a little olive oil. Season the steaks all over with salt and place on griddle for 3 minutes.
Discard the mushroom stalks, then place the caps on the griddle pan beside the steak. Allow to cook for 4 minutes, turning occasionally.
Peel the onion, slice into rings .25 inch think and place on the griddle to fry.
Flip the steaks and cook for a further 3 minutes. Preheat the oven on broiler high.
When the onion is cooked, transfer it to a small bowl and dress with the Worcestershire sauce. Put a place on top of the bowl so they keep cooking.
Roughly chop the thyme on a board and place the steaks on top to rest.
Deglaze the griddle with the Guinness and add this liquid and the mushrooms to the onions. Stir to combine.
Meanwhile slice the ciabatta in half lengthways and place under the grill to toast.
Once toasted spread each side with the mustard mayonnaise.
Roll the steak in the thyme and season with pepper. Slice and place on one sid of bread, then spoon over the mushrooms and onions. Top with watercress, blue cheese and other piece of bread. Slice in half before serving.