1). Position rack in top third of the oven and preheat broiler for baking. Now you can also grill or stewed the meat. You can also subsitute the beef for chicken, pork or seafood. Just make sure you measure the portions.
2). Whisk lemon juice, oil, mustard, salt and pepper in a large bowl. Place in a shallow dish and pour half the dressing overtop. 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. Transfer the meat to a broiling pan. Broil for 5 minutes on each side for medium-rare , or until it reaches dsired doneness.
4). Transfer to a cutting board. Let rest for 3 minutes, then slice thinly against the grain. Mix the meat with the salad and fill each pita. Serve immediately.