Preheat oven to 375 degrees. Place chicken breasts in the freezer. Freeze until very cold, about 10 minutes.
While the chicken chills, cut sweet potatoes in to fries. (I like to leave the skin on.
Once the chicken is cold, use a sharp knife to very thinly slice the meat. Add chicken, 1/3 cup lemon juice, 1 tbsp olive oil, rosemary, oregano and 2 garlic cloves to a large ziplock bag. Seal and massage the marinade in to the meat. Place in the fridge for 15 minutes.
While the chicken marinates, toss the sweet potato fries with corn starch. Once the fries are completely coated, toss with 2 tablespoons of olive oil. Evenly spread the fries out on to a large rimmed baking sheet. Do not overlap fries or they will steam. If your pan isn't big enough to to lay the fries flat in one layer, use two. Bake in the oven for 12-15 minutes or until the bottom starts to brown. Use a spatula to toss the fries. Sprinkle with garlic powder and 1/2 teaspoon salt. Spread back on pan and bake another 5-7 minutes until golden brown. Let fries cool slightly on baking sheet. When they get out of the oven, they may not seem crisp, but after they sit for a minute or two, they will crisp up!
To make the tzatziki, stir the cucumber, yogurt, remaining tablespoon lemon juice, 1 clove garlic, 2 teaspoons olive oil, feta and 1/4 teaspoon salt together. Season to taste with salt and pepper.
Once the chicken has marinated, dry with paper towels. Sprinkle evenly with remaining salt. Heat a large non-stick skillet to a medium-high heat. Once the pan is hot, add 3/4 tablespoon of olive oil. Add half of the chicken and cook until golden brown and cooked through. Transfer to a plate. Repeat with remaining chicken.
To assemble: Warm pita in the microwave for 35 seconds. Pile meat and a handful of french fries in pita. Dollop tzatziki on top. If desired, sprinkle with feta cheese.