1/2 cup fresh lime juice
1/4 cup chopped fresh cilantro
2 teaspoons brown sugar
2 tablespoons Thai fish sauce
2 tablespoons chile paste with garlic
2 garlic cloves, minced
1 (1 1/2-pound) flank steak, trimmed
1 1/2 cups vertically sliced red onion
4 plum tomatoes, each cut into 6 wedges
6 cups torn romaine lettuce
1 1/4 cups thinly sliced English cucumber
2 tablespoons chopped fresh mint
Prepare grill or broiler.
Combine first 6 ingredients, stirring until sugar dissolves; set half of lime mixture aside. Combine other half of lime mixture and steak in a large zip-top plastic bag; seal. Marinate in refrigerator 10 minutes, turning once. Remove steak from bag; discard marinade.
Place steak on grill rack or broiler pan coated with cooking spray; cook 6 minutes on each side or until desired degree of doneness. Let stand 5 minutes. Cut steak diagonally across grain into thin slices.
Heat a large nonstick skillet coated with cooking spray over medium-high heat. Add onion; sauté 3 minutes. Add tomatoes; sauté 2 minutes. Place onion mixture, lettuce, cucumber, and mint in a large bowl; toss gently to combine. Divide salad evenly among 6 plates. Top each serving with 3 ounces steak; drizzle each serving with 1 tablespoon reserved lime mixture.