1. Combine first 8 ingredients in a small bowl, stirring with a whisk. Place steak and 1/4 cup onion mixture in a large zip-top plastic bag; seal. Marinate in refrigerator at least 2 hours or overnight, turning bag occasionally.
2. Combine remaining 1/4 cup marinade, 1/2 cup water, peanut butter, and cornstarch in a small saucepan. Bring to a simmer; cook 1 minute, stirring frequently. Remove from heat; keep warm.
3. Remove steak from bag, discarding marinade. Heat a large nonstick skillet over medium-high heat. Add beef to pan; cook 2 minutes on each side or until desired degree of doneness. Serve with sauce.