In a large bowl mix together olive oil, lemon juice, minced garlic, Dijon mustard, dried oregano, salt and pepper. Add the chicken and toss to coat. Place in the fridge to marinate for a minimum of one hour (no longer than 8 hours).
Preheat the oven to 425 F.
Once marinated, spray the vertical gyro spit with nonstick spray. Stack the chicken thighs on top of each other, rotating each one opposite from the last. Add 1 cup of chicken stock to the base of the pan.
Transfer the pan to the preheated oven and bake for 90 minutes or until fully cooked through.
Remove from oven and spoon sauce from the bottom of the pan on top of the meat for extra flavor. Slice chicken into pieces right off the spit before serving.
To assemble gyro, spread tzatziki generously over lightly toasted pita. Top with desired amount of sliced chicken, tomato and red onion. Fold in half like a taco and enjoy!