For the Fish Croquettes:,1. Prepare the Fish Mixture: In a large bowl, combine the flaked fish, breadcrumbs, grated Parmesan cheese (if using), onion, garlic, egg, parsley, lemon juice, salt, and pepper. Mix well until the ingredients are fully combined and the mixture holds together.,2. Shape the Croquettes: With your hands, form the fish mixture into small, round or oval croquettes, about 1 1/2 inches in diameter. Roll each croquette in breadcrumbs to coat evenly.,3. Fry the Croquettes: Heat olive oil in a large skillet over medium heat. Once the oil is hot, carefully place the croquettes in the skillet and fry them for 2-3 minutes per side, or until they are golden brown and crispy. Once cooked, transfer the croquettes to a paper towel-lined plate to drain any excess oil.,For the Sauce:,4. Prepare the Sauce: In a small bowl, whisk together the Greek yogurt, lemon juice, olive oil, Dijon mustard (if using), salt, and pepper until smooth. Taste and adjust seasoning if needed.,For the Pitas:,5. Prepare the Pitas: Warm the pitas in the oven for a few minutes or in a skillet over low heat for about 1 minute on each side. This step is optional but helps make the pitas more pliable and delicious.,6. Assemble the Pitas: Open the pita breads and gently stuff each one with a few fish croquettes. Top with fresh lettuce or greens, sliced tomatoes, cucumber slices, and red onion rings.,7. Add the Sauce: Drizzle the prepared yogurt sauce over the top of the fillings in the pita. Garnish with fresh parsley, if desired.,Serve: Serve the pitas immediately while the croquettes are still warm.