Combine the lemon juice, oil, Oregano and Garlic Granules in a medium bowl. Add the tuna, tomato, red onion and feta cheese, season to taste and toss lightly.
Cover and refrigerate for at least 1 hour to allow the flavours to develop. Bring back to room temperature for approx. 30 minutes before serving.
Warm the pitta breads under a low-heat grill. Remove and slice in half. Open carefully and line with spinach leaves.
Spoon in the tuna mixture and serve immediately.