1. Position rack in upper third of oven; preheat broiler.
2. Whisk lemon juice, oil, mustard, salt and pepper in a large bowl. Place steak in a shallow dish and pour half the dressing over it. Let marinate at room temperature, turning once, for 10 minutes.
3. Meanwhile, prepare the salad by adding lettuce, cucumber and tomato to the remaining dressing in the bowl; toss to coat.
4. Transfer the meat to a broiling pan. Broil for 5 minutes on each side for medium-rare, or until it reaches desired doneness. Warm pitas on the bottom rack of the oven while the steak is broiling.
5. Transfer to a cutting board, let rest for 3 minutes, then slice thinly against the grain.
6. Mix the meat with the salad and fill each pita. Serve immediately.

Serves 4.

NUTRITION INFORMATION: Per serving: 410 calories; 15 g fat (3 g sat, 8 g mono); 65 mg cholesterol; 37 g carbohydrate; 33 g protein; 6 g fiber; 529 mg sodium. Vitamin A (70% daily value), Selenium (66% dv), Vitamin C (45% dv), Folate (29% dv), Iron (25% dv), Magnesium (20% dv).