To Make the Tzatziki: In a small mixing bowl, combine the Greek yogurt, squeezed grated cucumber, minced garlic clove, lemon juice and dill, stirring until thoroughly mixed. Cover and refrigerate for at least 30 minutes so that the flavors meld together. The meatballs also need to chill before cooking, so it’s good to prep them together so you can chill them all at once.
To Make the Meatballs: In a mixing bowl, add the egg, panko, garlic, dried onion, salt, pepper, oregano, basil, and rosemary, and stir together with a fork until well combined. Add the chopped spinach, crumbled feta, and ground chicken and stir the chicken into the other ingredients with a fork until just mixed.
On a clean surface, divide the meat mixture into 16 evenly sized piles. Mist your clean hands with cooking spray and roll each meat pile into a meatball. Place the meatballs into a single layer in a sealed container and refrigerate them for at least 30 minutes to firm them up.
When the meatballs are chilled, drizzle the olive oil into a large sauté pan and bring over medium heat. When the oil is hot, add the meatballs in a single layer with a bit of space in between them. Cook, flipping them every couple minutes to brown all sides, for 8-10 minutes. Cover the pan with a lid and reduce the heat to low. Cook covered, removing the lid once or twice to stir the meatballs and then covering them again, for about 5 minutes until the meatballs are cooked through.
To Make the Sandwiches: Place 1-2 lettuce leaves in the center of each pita and top with 1-2 slices of tomato. Top that with 3 tablespoons of tzatziki and four meatballs on each sandwich and serve.