2 Tbsp Balsamic Vinegar
1/4 Tsp Dijon Mustard
16 Oz Tuna
1 Tbsp Olive Oil
4 Tsp Lemon Juice
2 Tsp chopped Chives
1/3 Tbsp chopped Shallots
1 Dash Salt
1 Dash Pepper
2 Cup, chopped Watercress
Combine vinegar and mustard in small bowl; whisk in 4 tablespoons olive oil.
Season dressing to taste with sea salt and pepper.
Cover and chill.
Place 1 sheet of plastic wrap on damp work surface (to prevent plastic from slipping).
Arrange 1/4 of tuna squares 1 inch apart in circle on plastic.
Cover with second sheet of plastic.
Using flat side of mallet, gently pound tuna squares until they are very thin and form an 8- to 9-inch solid round.
Place tuna round, still in plastic, on plate.
Form 3 more rounds with remaining tuna.
Refrigerate tuna carpaccio at least 30 minutes and up to 4 hours.
Peel 1 plastic sheet from each tuna carpaccio round; invert onto plates and peel off remaining plastic.
Brush each with 1 tablespoon olive oil, then sprinkle with lemon juice, chives, shallot, sea salt, and pepper.
Toss watercress and 2 tablespoons dressing in medium bowl; season to taste with sea salt and pepper.
Mound watercress atop and serve.