For the steak: Sprinkle the steak liberally with olive oil, salt and pepper; make sure the entire steak is covered. Wrap lightly and place in the refrigerator for 12 hours.
Get the grill hot 1 hour before cooking (a charcoal or wood fire, preferably with hardwood); the embers must be burned down and glowing and there should be no flames at all. Remove the steak from the refrigerator and leave at room temperature for the same hour that the grill is getting ready.
When ready to grill, place the steak on the center of the hot grill and rotate left and right to get proper grill marks. Do this for both sides, about 10 minutes. When this is done, move the steak to a cooler part of the grill and cook to rare, 25 to 30 minutes. You will have to test it with your fingers by pressing gently. When done, place the steak on a rack set inside a sheet pan so the steak does not sit in its own juices. Then place a knob of French butter on top and lightly tent it with tin foil to rest for at least 15 minutes before carving.
For the romaine marmalade and watercress: In a blender or food processor, combine the watercress, romaine, garlic, cilantro, parsley, lemon juice, salt and pepper. Add the oil and puree until smooth. Set aside. Yield: 2 cups.
To serve, slice each steak into 3 thick, equal-size portions. Spoon equal portions of the watercress-romaine puree onto each of 6 warm plates. Arrange the sliced steak on each plate. Strain the steak pan juices and pour them on top of the meat.