Mix half of chopped cilantro, butter, chopped green onions, rice vinegar, red miso, minced ginger, and soy sauce in small bowl. Set miso sauce aside.
Sprinkle both sides of sirloin steak with cracked coriander seeds and ground black pepper, pressing to adhere. Heat Asian sesame oil in medium nonstick skillet over medium high heat. Add sirloin steak; cook to desired doneness, about 4 minutes per side for medium rare.
Transfer steak to work surface; reserve skillet. Slice steak; arrange on plates.
Add miso sauce and sake to skillet; boil mixture until slightly thickened and reduced to 1/4 cup, whisking often, about 1 minute.
Spoon miso butter sauce over steaks. Sprinkle with remaining chopped cilantro.