Place tomatillos and chiles in a medium saucepan and add enough water to cover them.
Bring to a boil and cook 5 to 8 minutes or until tomatillos are slightly tender, stirring occasionally, then drain.
Place tomatillo mixture, cilantro leaves, and garlic in a food processor or blender. Cover; process until finely chopped but not puréed. Season with salt and black pepper as needed and set aside.
Press black pepper evenly onto thawed flat iron steaks. Heat a large nonstick skillet over medium until hot.
Place steaks in skillet and cook 13 to 15 minutes, turning once, or until temperature reaches 130 degrees Fahrenheit, as per a meat thermometer, for medium-rare doneness. Once doneness is achieved, allow steaks to rest for 5 minutes until slicing. For a different desired doneness, refer to our helpful Steak Cooking Chart.
Cut steaks across the grain into thin slices, season with salt and black pepper to taste, and spoon tomatillo sauce over slices.
Serve in warmed flour tortillas and garnish with additional cilantro.