Season the raw tuna with salt and crust on all sides with sesame seeds.
Add sesame oil to the hot pan and quickly sear the tuna on all sides, keeping it rare. Set the cooked tuna aside when finished, resting it on a paper towel.
Place cabbage, watercress, oranges, chestnuts, cilantro, wontons, scallions and dressing in a large mixing bowl and toss gently. Plate neatly in the center of a large dish.
Slice the tuna thin and arrange on top of the salad.