Place tomatillos and chiles in medium saucepan; add enough water to cover. Bring to a boil; cook 5 to 8 minutes or until tomatillos are just tender, stirring occasionally. Drain.
Place tomatillo mixture, 1/4 cup cilantro and garlic in food processor or blender container. Cover; process until finely chopped, but not pureed. Season with salt and black pepper, as desired. Set aside.
Meanwhile press 1/4 teaspoon black pepper evenly onto beef Flat Iron Steaks. Heat large nonstick skillet over medium heat until hot. Place steaks in skillet; cook 11 to 14 minutes for medium rare (145°F) to medium (160°F) doneness, turning occasionally.
Carve steaks across the grain into thin slices; season with salt and black pepper, as desired. Spoon tomatillo sauce over steak. Serve in flour tortillas, if desired. Garnish with additional cilantro, if desired.
Nutrition information per serving: 221 Calories; 108 kcal Calories from fat; 12 g Total Fat; 4 g Saturated Fat; 5 g Monounsaturated Fat; 7 mg Cholesterol; 67 mg Sodium; 6 g Total Carbohydrate; 1.7 g Dietary Fiber; 22 g Protein; 3 mg Iron; 4.8 mg NE Niacin; 0.4 mg Vitamin B6; 5 mcg Vitamin B12; 7.9 mg Zinc; 32.7 mcg Selenium;