Trim fat from steak. Thinly slice steak diagonally across grain into 24 slices. Combine onions and next 7 ingredients in a large zip-top plastic bag. Add steak to bag; seal bag. Marinate in refrigerator 8 hours, turning occasionally. Remove steak from bag, discarding marinade.
Drain pineapple, reserving 3/4 cup juice and 24 pineapple chunks; reserve remaining pineapple chunks and juice for another use. Combine 3/4 cup juice, peanut butter, and next 4 ingredients in a medium saucepan, stirring well. Using cheesecloth or fingertips, squeeze juice from grated ginger into saucepan; discard pulp. Bring to a boil over medium-high heat; reduce heat, and simmer 1 minute, stirring constantly. Remove from heat; let cool.
Thread 1 chunk of pineapple and 1 slice of steak onto each skewer. Coat grill pan or nonstick skillet with cooking spray; place over medium-high heat until hot. Add skewers to pan, and cook 1 minute on each side or until desired degree of doneness. Serve with peanut sauce.