1. In a non-reactive bowl, combine the sugar, vinegar and sumac with a pinch of salt. Whisk together until sugar has dissolved. Fold in the onions, sprinkle with a pinch of salt, toss to coat thoroughly. This will help to draw out the moisture from the onions. Cover let stand at room temperature for 1 hour, then refrigerate for 2 hours before using.
2. In another non-reactive bowl, place the ground beef, garlic, dry Greek oregano, parsley, Kefalotyri cheese, the soaked bread with the milk, cooked onion, egg and salt and pepper to taste (work thoroughly, with clean hands). Combine the mixture for approximately 1 minute until well incorporated and slightly sticky add the bread crumbs to tighten the mixture should be moist to the touch. Cover and chill in the refrigerator for 1 hour.
3. Remove from refrigerator and form six – 5 ounce oval burgers, approximately 6” long and 2 ½ “wide. Prepare standard gas grill to medium heat or charcoal grill to light ash stage. Rub the grill with olive oil or non stick spray. Then brush the burgers with the extra virgin olive oil, season with salt and pepper on both sides.
4. Place on the grill and grill for 2- 3 minutes until grill marks are visible. Turn the burgers to make cross grill mark, cook for another 2-3 minutes, turn and repeat this process. The burger will be cooked through and will no longer be pink. Set aside to rest 1 minute.
5. Lightly grill pita bread on both sides 1-2 minutes until soft and pliable
To make the sandwich
Put down a ¼ piece of parchment paper, place the warm pita down on the parchment paper, then 2 tablespoons of tzatziki, the Bifteki, top with 3 slices of tomato, 1 tablespoon of marinated onions, shredded romaine. Fold over pita and wrap the pita in paper in a cone fashion so that the top half is exposed and serve.
1. Line a colander with a double layer of cheesecloth. Place the colander over a bowl deep enough to catch draining juices without the juices touching the bottom of the colander. Set aside.
2. Using a box grater, grate the cucumber through the medium holes into a mixing bowl. Add a pinch of salt and toss to combine. Transfer the cucumber to the colander and allow the juices to drain for 1 hour. Pull the cheesecloth up and twist the ends together to squeeze any remaining juice from the cucumber. Transfer the well-drained cucumber to a clean non-reactive container with a lid.
3. Add the yogurt, garlic, lemon juice, mint and dill. Season with salt and pepper to taste and toss to combine. Cover and refrigerate for 8 hours or overnight before serving.
4. Serve chilled with warm pita bread triangles.