In a large mixing bowl, combine the ground chicken, breadcrumbs, crumbled feta cheese, parsley, diced onion, minced garlic, dried oregano, lemon juice, salt, and pepper. Mix until everything is well incorporated.
Preheat your oven to 400°F (200°C).
Form the mixture into small meatballs, about 1 inch in diameter, and place them on a baking sheet lined with parchment paper.
Bake the meatballs in the preheated oven for 20-25 minutes, or until they are cooked through and golden brown.
Meanwhile, warm the pita breads in a skillet or wrap them in foil and heat them in the oven for about 5 minutes.
Once the meatballs are ready, assemble the pitas: cut each pita in half to create pockets, then fill each pocket with a few meatballs, a spoonful of tzatziki sauce, diced cucumber, halved cherry tomatoes, and sliced red onion.
Serve the stuffed pitas warm, with extra tzatziki sauce on the side if desired.