To prepare steak, combine first 7 ingredients in a large zip-top plastic bag; add steak. Seal and marinate in refrigerator 8 hours or overnight, turning bag occasionally. Remove steak from bag, reserving marinade.
Place steak on grill rack coated with cooking spray; grill 5 minutes on each side or until desired degree of doneness. Let stand 10 minutes. Cut diagonally across grain into thin slices.
Combine reserved marinade and cornstarch in a saucepan. Bring to a boil; cook 1 minute, stirring constantly.
To prepare potatoes, place potatoes and garlic in a large Dutch oven; cover with water. Bring to a boil. Reduce heat; simmer 30 minutes or until tender. Drain.
Return potatoes and garlic to pan, and place over medium heat. Add sour cream, milk, butter, salt, and 1/4 teaspoon pepper. Mash potato mixture to desired consistency with a potato masher. Stir in chopped chives. Mound 3/4 cup potatoes on each of 8 plates; arrange 3 ounces steak around each serving of potatoes. Drizzle 1 tablespoon sauce on each plate; sprinkle with chive pieces, if desired.