Cut the pork into 1-inch size pieces. Set aside in a bowl.
In a medium bowl combine oregano, lemon juice, extra virgin olive oil, salt, and pepper to make the marinade. Take out 3 tablespoons of the marinade drizzle it over the pork pieces and toss.In the remaining marinade add the onion slices and toss. Set aside.
Heat a cast-iron skillet over medium high heat. Add 2 teaspoons of olive oil to the skillet and half of the pork cubes. Sauté for about 5 minutes or until meat is golden brown and cooked. Transfer to a bowl. Repeat with the remaining pork.
Strain the onion slices from the marinade, drizzle the remaining marinade over the cooked pork pieces.
To Serve: Arrange the pork, tomatoes, onion slices, lemon wedges, and pita bread on a large platter. Serve with Tzatziki and mint.