Cut tenderloin into bite-size chunks and put into a container with remaining marinade ingredients.
Swish everything around to distribute, put in fridge and forget for a day (this long marination is not absolutely necessary- but longer means more flavour. If you haven't the time, just do what you can).
Tzatzki
Combine all ingredients and refrigerate
Grill meat
When ready to BBQ, skewer meat onto metal or wooden skewers (remember to soak the wooden skewers in water for at least 1/2 an hour first) and grill over direct heat until the meat is nicely browned and internal temperature is about 150 deg. Do not overcook.
As you are barbecuing the meat, brush as many pita breads as there are souvlaki generously with olive oil and put on the bbq to crisp slightly or alternatively, into a hot frying pan, and cook about 30-45 seconds per side or until starting to brown and give off a yummy,'toasty' smell.
Place grilled meat on pita with tomato and onion. Add dollops of Tzatzki sauce and serve.