In a small bowl, combine cilantro, parsley, oregano, onion, garlic and hot pepper flakes, if desired. Stir in lemon juice and vinegar. Slowly drizzle in olive oil, stirring constantly until combined. Season with adobo. Cover and refrigerate until ready to use (can be kept in refrigerator for up to 3 days).
Prepare grill to medium-high heat, or heat grill pan over medium-high heat. Sprinkle steak on both sides with sazon and adobo. Cook steak, flipping, until golden brown, about 6 minutes for medium rare.
Transfer steak to serving plates; top with chimichurri sauce.