In a bowl, combine the thyme, oregano, onion powder, sugar, pepper, paprika powder, garlic powder, and celery salt.
Rub the spice mixture evenly over the pork steak slices and refrigerate for at least 30 minutes.
Thread the spiced meat slices onto skewers, evenly distributing and alternating with the half onion.
Preheat the grill to medium-high heat. Place the skewers on the grill and cook for about 2 hours, until the internal temperature of the pork reaches 63°C (145°F).
Using an electric knife, slice the meat from the skewers. Serve with flatbread and salad with tzatziki.