In a resealable plastic bag or shallow dish, combine 3 tablespoons of lemon juice, soy sauce, minced garlic, and 1 teaspoon of oregano. Add the chicken breasts and coat thoroughly. Seal the bag or cover the dish, and allow the chicken to marinate at room temperature for 30 minutes, or refrigerate overnight for deeper flavor.

In a small bowl, combine the chopped tomatoes, crumbled feta cheese, 2 teaspoons lemon juice, olive oil, and the remaining teaspoon of oregano. Stir gently to combine, and let it sit at room temperature for approximately 30 minutes to allow the flavors to meld.

In a separate bowl, whisk together the yogurt, garlic, chopped walnuts, and olive oil until smooth. Cover and refrigerate until ready to use.

Preheat the grill to medium heat. Lightly spray the grates with olive oil spray to prevent sticking. Grill the marinated chicken breasts for 8 to 10 minutes, turning once, until the internal temperature reaches 165°F and the meat is no longer pink. Simultaneously, place the red onion rings on the grill and cook for 8 to 10 minutes, or until softened and lightly charred.

Once the chicken and onions are cooked, place the pita breads directly on the grill grates. Toast each side for approximately 2 minutes, or until warm with visible grill marks. Remove from the grill.

Allow the chicken to rest for several minutes, then slice thinly across the grain. To assemble each pita, layer several slices of grilled chicken, a few grilled onion rings, one tablespoon of the tomato-feta mixture, chopped romaine lettuce, kalamata olives, one to two slices of crispy bacon, a spoonful of yogurt-walnut sauce, and alfalfa sprouts if using. Season with salt and pepper to taste. Fold the pita and serve warm.