In a bowl whisk together the olive oil, lemon juice, and dijon mustard. Mix in the tuna, celery, cucumber, red onion, and olives; season with salt and pepper to taste.
Warm the pita bread in the oven on a baking sheet for 1 -2 minutes.
Layer the pita with cheese, tomatoes seasoned with salt and pepper, spinach, and the tuna mixture. Fold in half and serve with chips and pickles.