Preheat the oven to 400°F (200°C). In a large mixing bowl, combine the ground turkey and ground beef. Add the egg, bread crumbs, Dijon mustard, basil, oregano, salt, and pepper. Mix thoroughly until all ingredients are evenly distributed. Form the mixture into small, firm meatballs, about 20 to 24 in total. Arrange them on a broiler pan coated lightly with cooking spray, ensuring they are evenly spaced.

Transfer the pan to the oven and bake the meatballs for 15 to 18 minutes, or until they are browned and cooked through in the center.

While the meatballs are in the oven, prepare the tzatziki sauce. In a small bowl, mix the finely chopped cucumber with Greek yogurt, garlic, and dill. Season with salt and pepper to taste. Stir well, then cover and refrigerate until ready to serve.

Prepare the toppings by chopping the vegetables and crumbling the feta. When ready to serve, warm the pita bread slightly if desired. Fill each pita with green leaf lettuce, a few hot meatballs, and a generous spoonful of tzatziki. Add red onion, tomatoes, cucumber, bell pepper, and crumbled feta to taste.

The meatballs can also be frozen for later use. Let them cool completely before transferring to a large resealable freezer bag. Store for up to a few weeks and reheat for a quick, flavorful meal any night of the week.