Place butter and miso in a large bowl, stirring to combine. Sprinkle steak evenly with pepper and salt. Heat a large skillet over medium-high heat. Coat pan with cooking spray. Add steak to pan; cook 2 minutes on each side or until desired degree of doneness. Add steak to butter mixture; toss to coat. Sprinkle steak with green onions.
Stir rice vinegar and oil into rice. Serve rice with steak mixture.