1. Preheat grill to medium-high heat.
2. Sprinkle the steak evenly with 3/8 teaspoon salt and 1/4 teaspoon pepper. Sprinkle red onion with 1/8 teaspoon salt. Coat green onions with cooking spray. Place steak on grill rack coated with cooking spray; grill 3 minutes on each side or until desired degree of doneness. Let stand 10 minutes. Cut steak diagonally across grain into thin slices. Grill red onion 5 minutes on each side or until tender. Grill green onions 1 1/2 minutes on each side; cut into 3-inch pieces.
3. Place potato in a saucepan; cover with cold water. Bring to a boil; cook 10 minutes or until tender. Drain. Return potato to pan; add 1/4 teaspoon salt, sour cream, milk, and butter. Mash to desired consistency. Garnish with 1/4 teaspoon pepper.