In a medium saucepan, heat broccoli and 1/4 cup water to boil and steam for 1 minute. Remove from heat and cool under cold running water. Drain and set aside.
Measure mayonnaise into a large serving bowl. Slowly add buttermilk, whisking until smooth. Whisk in vinegar, sugar, celery seed, pepper and salt. Stir in rice, broccoli, onion and raisins, until well coated with dressing.