Brine the chicken. In a large mixing bowl, stir the salt (2 tablespoons) and water(4 cups) until the salt has completely dissolved. (I usually use 1/2 cup of boiling water and dissolve the salt. Then add 3 1/2 cups of cold water so it doesn't cook the chicken. Make sure the water is room temp before adding chicken!) Place chicken (1 pound) in brine and refrigerate for 30 minutes - 3 hours.
Pickle the veggies by adding the apple cider vinegar (1/2 cup), salt (1 teaspoon), and erythritol (1 tablespoon) together in a small saucepan over medium-high heat. Whisk until the erythritol completely dissolves. Place sliced red onions (1 sliced) in a heat-safe bowl, and pour the boiling vinegar over the top. Place a plate over the top and allow the veggies to rest like this for a minimum of 30 minutes--making sure to stir occasionally so the veggies on top and bottom pickle evenly.
Make the naan. This is a great time to make the naan if you haven't already done so. Wrap cooked naan in a kitchen towel while you prep everything else. You can reheat the naan by wrapping it in damp paper towels and tossing it in the microwave for 15-second intervals.
Marinade. In a small bowl, whisk together the olive oil (1/3 cup), lemon juice (1/4 cup), parsley (2 tablespoons), lemon zest (1 teaspoon), erythritol (1 teaspoon), and oregano (1 teaspoon). Remove chicken from brine and pat completely dry with paper towels. Discard brine. Place chicken in a large mixing bowl, and pour half the marinade over the top (Set the other half of the marinade aside.). Toss to evenly coat the chicken. Thread chicken pieces onto skewers.
Cook chicken, either on a hot grill or over the stove in a grill pan for 5-7 minutes on each side. You'll know the chicken is done once it reaches an internal temp of 165 F. Use a fork to push the chicken off the skewers into the bowl with the reserved marinade and toss to coat.
Assemble. Place naan on a piece of foil, then spread some tzatziki, chicken, veggies, and feta. Serve warm and enjoy!