AT HOME (UP TO 1 DAY AHEAD)
Slice brussels sprouts very thinly. Pack sprouts, red onion, and tangerine segments in separate resealable plastic bags. Chill.
Put oil, lemon juice, 1/4 cup cheese, the salt, and pepper in a jar with a tight-fitting lid. Chill.
AT THE PARTY
Put sprouts, onion, and tangerines in a large salad bowl. Shake dressing well and pour over salad; toss very well. Sprinkle with remaining 2 tbsp. cheese.