In a medium bowl, whisk together all the the ingredients for the marinade, except the olive oil. Then slowly stream in the olive oil to emulsify. Add the steak; toss to coat and let marinate for 15 minutes.
Lightly oil the grill grates. Remove steak from the marinade and pat dry. Place on grill and cook for 4 minutes per side for medium-rare (an internal temperature of 135 degrees F.) Transfer to a cutting board and let rest for 5 minutes. Slice thinly across the grain.
Divide the lettuce and tomatoes among plates; drizzle with any juices that accumulated from the cutting board, and top with the steak and Tzatziki Sauce. Serve and enjoy!
Nutritional information given is an automatic calculation and can vary based on the exact products you use and any changes you make to the recipe. If these numbers are very important to you, I would recommend calculating them yourself.