In a medium bowl, add the chicken, oregano, salt, pepper, red pepper flakes (if using), and garlic. Use your hands to mix to combine well. Divide into 4 (4 oz) or 8 (2 oz) portions (depending on the size of your tortillas, see note).
To a tortilla, add one portion of the chicken and press it out into a very thin even layer that coats the entire tortilla. Repeat with the remaining tortillas.
In a large skillet over medium-high heat, warm the oil. Once the skillet is hot, place the tortilla, chicken-side-down. If your tortillas are small, you can do 2 at a time or however many you can fit in the skillet. Press/smash it with a spatula and let it cook, pressing them down every minute or so, for 3-5 minutes or until the chicken is cooked and browned.
Flip it over and top the chicken with marinara (spread it out), mozzarella, and parmesan. Reduce the heat to low, cover with a lid, and let it cook for another 2-4 minutes or until the cheese is melted and tortilla is crispy..
Meanwhile, toss the greens with the juice from half of a lemon and a sprinkle of salt (about ¼ tsp) and pepper. Toss to coat.
Serve the tacos topped with the dressed greens.