Combine the pork, red onion, bell pepper, lemon zest, lemon juice, 1/3 cup olive oil, garlic, oregano, rosemary, 2 teaspoons salt, and 1 teaspoon black pepper in a large (1-gallon) plastic storage bag. Press out the air, seal, and set aside at room temperature for 30 minutes or refrigerate for up to 12 hours.
Preheat one very large (14-inch) or two medium (10-inch) dry cast-iron skillets over high heat for 3 minutes. Add the pork and vegetables, including the marinade, spread out, and cook without stirring for 3 minutes. Continue to cook over high heat for 7 to 8 minutes, stirring occasionally, until the pork is cooked through but still slightly pink in the middle. Don't overcook the pork or it will be dry!
Place 1 warmed pita on each plate. Spoon the pork and vegetables on one half of the pita and place 2 rounded tablespoons of Radish Tzatziki on the other half. Sprinkle with mint, lemon juice, and salt and serve hot.
Grate the radishes on a box grater. Place the radishes in a paper towel and squeeze out the liquid. In a medium bowl, combine the radishes, garlic, yogurt, olive oil, lemon juice, scallions, mint, 1 1/2 teaspoons salt, and 3/4 teaspoon pepper. Cover, and refrigerate for at least 30 minutes or for up to 12 hours.