At least one hour or as early as one day before you prepare the meat, prepare the relish and tzatziki:
Prepare the cucumber tomato relish:
Toss the olive oil, vinegar, tomatoes, cucumbers and onions in a bowl and adjust the seasoning if necessary. Cover and let sit at least 1 hour to marry the flavors.
For the tzatziki:
Seed and then finely dice the cucumber and place in a fine-meshed strainer or cheese cloth fitted over a bowl. Salt the cucumber and let sit 10 minutes. Then squeeze out any excess moisture. Add the strained cucumber into a bowl with the yogurt, feta, garlic and mint. Adjust the seasoning if necessary.
The day of the tacos, start about 20 minutes before ready to serve:
For the meat:
In a large skillet, heat the oil over medium-high heat. Add the ground meat, sprinkle with salt and pepper and cook until the meat gets nice and golden brown, 10 to 12 minutes. With a slotted spoon, remove the meat and reserve. Lower the heat to medium and saute the onions in the lamb juices until soft, about 5 minutes. Add the oregano and garlic and cook until fragrant. Season. Add the tomato paste and cook until it caramelizes. Deglaze with the red wine and scrape any bits up that have adhered to the pan. Add the meat back in, stir and keep warm.
For the pitas:
In a cast iron or other heavy skillet, drizzle a small amount of olive oil and heat to medium high.
Place pita in pan and heat each side for just about 20 seconds.
Serve buffet style with extra feta, if desired.