Preheat oven to Hi-Broil. Line a sheet pan with Reynolds Wrap® Heavy Duty Aluminum Foil. Blot steak with paper towels and sprinkle salt over both sides. Place on pan under broiler 10-12 minutes for medium-rare, depending on thickness of steak. Transfer to carving board and let rest 8 minutes, covering loosely with Reynolds Wrap® Heavy Duty Aluminum Foil to keep the temperature while the meat is resting.
Adjust spiciness of the chile-churri by using more or less seeds from the serranos. More seeds equals more heat. In a food processor, process serranos, garlic, oregano, salt, parsley, cilantro and zests until finely chopped. Transfer to a bowl and stir in citrus juices and olive oil. Slice flank steak thinly against the grain. Serve with chile-churri drizzled on top.