The night before, lightly sprinkle the steak on both sides with salt. Allow to sit, uncovered, in your fridge overnight.
One hour prior to cooking, remove the steak from the fridge and allow to come to room temperature. Preheat a grill to medium-high heat.
Sprinkle the steak on both sides with salt and pepper. It is important to use more seasoning than needed, as the steak is very thick and much of the seasoning will fall off during the cooking process.
Grill the steak on one flat side, 5 to 8 minutes. Stand the steak up on one short side and grill for 1 to 2 minutes. Turn over and grill on the other flat side, 5 to 8 minutes. Stand it up on the opposite short side and grill for 1 to 2 minutes. Move the steak to the cooler part of the grill and cook until the internal temperature reaches 125 degrees F for medium rare. Transfer to a plate and allow to rest until the internal temperature registers 130 degrees F, about 20 minutes.
Measure out 1 tablespoon of the steak juices that have settled onto the plate and put in a medium mixing bowl. Whisk in the mustard, shallots and vinegar. Toss the watercress in the dressing, along with the tarragon leaves and radish slices.
Place the salad on a plate with the steak. Drizzle the steak with the olive oil and aged balsamic vinegar. Finish with the sea salt.