Preheat the oven to 350°F. Mix together the celery salt, cumin, garlic salt, ground coriander, oregano, paprika, salt and black pepper on a plate. Roll the pork joint in the spice mix so that all the meaty sides are well covered (no need to cover the rind).
Heat 2 tbsp of the oil until hot in a dutch oven or large thick-based, oven proof pan. Sear the meat on all sides (except the rind side), then add the garlic and stock. Bring to the boil, then turn off the heat. Place a lid on and place in the oven to cook for 2 ½ – 3 hours – until you’re able to pull the meat away easily with a fork. Check the pork a couple times during cooking to ensure there’s always some liquid in there. If it starts to go dry, top up with half a cup of hot water from the kettle. When cooked, take out of the oven and remove the lid.
Next make the tzatziki. Peel the cucumber and slice in half, lengthways. Remove the seeds by scooping them out with teaspoon. Grate the cucumber with a hand grater, then squeeze all of the liquid out using clean hands. Place the squeezed out cucumber in a bowl and add the rest of the tzatziki ingredients. Mix together, then cover and refrigerate until ready to serve.
Carefully remove the meat from the pan and place on a chopping board. Discard the cooking liquid (it will be very salty). Discard the pork rind too. Pull the meat into small chunks using two forks.
Heat the remaining oil in a large skillet and add the pork. Fry on a high heat, turning a few times during cooking, until golden brown (about 6-8 minutes). You may need to do this in two batches, depending on the size of your pan.
Place the pitas on plates along with the lettuce, tomato and red onion. Top with the gyros meat and the cucumber tzatziki. Drizzle with sweet chili sauce before serving.
Notes
*It sounds like a lot, but there’s quite a lot of fat which renders down, and also the rind which is removed after cooking.