2) When the griddle is medium-hot, brush the grate with oil to keep the burgers from sticking. Place the burgers on the griddle and cook for 4 minutes. Turn the burgers over with a spatula and cook for another 4 to 6 minutes, until medium-rare, or cook longer if you prefer the meat more well done.
3) For the last 2 minutes of cooking time, place 15 grams of Gruyere on the top of each burger and close the griddle lid. Remove the burgers to a platter and cover with foil.
4) Slice the baps in half crosswise and toast the halves cut side down on the griddle. Divide the baby rocket among the 12 bottom baps, top each with a burger and finish with a slice of tomato and red onion. Cover with the top of the bap and serve hot with tomato ketchup.