In a bowl, combine 2 tablespoons canola oil with the pepper, salt and sugar. Add the cubed filet mignon.
Cover and refrigerate for 2 hours.
Meanwhile, combine all the ingredients for the stir-fry sauce and mix well.
Divide the meat in two. (You will cook it in two batches.).
Place a pan over high heat until hot. Add 4 tablespoons of canola oil, or enough to coat the bottom of the pan.
Place about half the beef in the pan so that none of the pieces overlap. (Be careful - the oil may splatter.) Cook 1 1/2 - 2 minutes on each side for medium doneness.
With the meat still in the pan, drain most of the oil, leaving about 1 tablespoon. Add half the garlic and stir. Add half the red and green onions and cook about 30 seconds. Add half the stir-fry sauce and cook 1 minute. Add half the butter and toss the ingredients in the pan until the butter is melted. Remove the beef and keep warm.
Repeat with the second batch of meat.
To Serve: Place the watercress in the center of a serving platter and spoon the beef over top.
Combine the salt and pepper in a small bowl and squeeze the lime juice into it to create a dipping sauce. Serve immediately.