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1
In 8-inch or 9-inch square glass baking dish, stir 1 tablespoon fish sauce and 1 teaspoon sugar.
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2
Add steak, turning to coat; marinate 15 minutes at room temperature or 1 hour in refrigerator, turning occasionally.
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3
Prepare charcoal fire or preheat gas grill for covered direct grilling over medium heat.
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4
Meanwhile, from limes, with vegetable peeler, remove peel in 2-inch by 3/4-inch strips.
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5
With sharp knife, cut enough peel crosswise into matchstick-thin strips to equal 1 tablespoon.
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6
Squeeze limes to equal 3 tablespoons juice.
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7
In small bowl, whisk lime juice, oil, crushed red pepper, black pepper, the remaining 1 tablespoon fish sauce, and 1 1/2 teaspoons sugar until blended.
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8
In large bowl, toss watercress, mint, cilantro, radishes, onion, and lime peel; cover and refrigerate until ready to serve.
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9
Place steak on hot grill rack.
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10
Cover grill and cook steak 10 to 15 minutes for medium-rare or until desired doneness, turning over once.
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11
Transfer steak to cutting board; let stand 10 minutes to set juices for easier slicing.
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12
Cut steak diagonally into thin strips.
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13
Add steak and dressing to watercress mixture and toss until well coated.
