Steak in a sack sandwich is intended to solve a problem. The problem is that cutting steak on a paper plate using a plastic knife is a pain in the butt. The knife does a poor job, you end up cutting through the plate, and you end up with flecks of paper in your steak. But if you're trying to avoid washing (or transporting) dishes what can you do? Answer: Bring a sharp knife, a cutting board, and pita bread. The recipe below features a Mediterranean marinade, but you can flavor the steak any way you prefer. I like sirloin for this, it's a good compromise on taste, tenderness, and price.
Combine marinade ingredients in a zippered plastic bag and marinate steak 12 to 16 hours in the refrigerator, turning occasionally.
Mix together tzatziki ingredients.
Cook the steak over a hot grill for 2 to 3 minutes per side depending on the thickness of the steak.
Remove steak from grill, tent with foil, and let rest 10 minutes.
Warm pita in a microwave, wrapped in a couple of paper towels, for 15 seconds.
Slice steak very thin across the grain and stuff into the pita along with tomato, onion, lettuce, and tzatziki.