Pop the onion rounds in a tray, drizzle over the vinegar and sprinkle over the sugar and salt. Chuck in the bay leaf and cover with foil.
Turn the Roccbox burner down to low and add the onion tray. Cook for 10-12 minutes and set to one side.
Turn the flame back to high and pre heat a cast iron griddle pan in Roccbox. Season and oil the steak.
Cook the steak in Roccbox for around 2-3 minutes. Remove the pan and brush both sides of the steak with the Dijon mustard. Continue to cook, turning and brushing with more mustard as you go for around 6-8 minutes. Remove the steak from the pan and pop it onto a plate to rest.
Toss the tomatoes with olive oil, paprika, salt and pepper and add to the hot pan.
Return to Roccbox until the tomatoes begin to break down. Chuck in the rosemary stalk and move around the tomatoes before setting aside in a bowl. Turn Roccbox down to its lowest setting.
Slice the ciabatta and rub the inside of the bread in the resting juices before toasting, crust side up in the same pan.
To build the sandwich, spoon over some mayonnaise, add the onions, squash over the tomatoes and top with watercress.
Slice the beef and add to the sandwich, pop the lid on and squish down!