In a large skillet, heat oil over medium-high heat; brown steak on both sides. Transfer to a 3-qt. slow cooker.
Add onion to same skillet; cook and stir 1-2 minutes or until crisp-tender. Add water to pan; cook 30 seconds, stirring to loosen browned bits from pan. Stir in remaining ingredients; return to a boil. Pour over steak.
Cook, covered, on low 6-8 hours or until meat is tender. Slice steak across the grain; serve with onion mixture.