Shred zucchini on the large side of a box grater into a strainer set over the sink or a bowl. Salt it and let it drain for 20 minutes. Press out excess liquid and transfer to a bowl.
While zucchini drains, in a shallow dish, combine about 3 tablespoons EVOO and the zest of 1 lemon, garlic, chili flakes, oregano and some salt. Halve and peel the red onion and grate a few tablespoons into dressing. Add chicken and turn to coat evenly in marinade. Finely chop about 1/2 cup of remaining red onion for the salsa and place in another bowl.
To the salsa bowl, add peppers, tomatoes, olives, parsley, pickled peppers and feta cheese. Add juice of 1 lemon to the salsa, drizzle with EVOO and season with salt and pepper.
In another bowl, combine drained zucchini with juice of 1 lemon, yogurt, cumin, dill and salt to taste.
Heat a cast-iron skillet or griddle over medium-high heat and cook chicken 3 minutes on each side or until browned and cooked through. Remove to cutting board, douse with remaining lemon juice.
Char pitas over open flame burner or toast in a dry, stainless skillet to blister and warm.
Fill pitas with tzatziki, lettuce, salsa, chicken and serve.