Soak red onion in cold water for 5 minutes. Steam florets for roughly the same period until just tender but firm.
Combine mayonnaise, cider vinegar, salt, and pepper, stirring well with a whisk. Add red onion, broccoli, almonds, and cranberries, then toss. Cover and chill for 1 hour before serving.
Note: The salad improves resting overnight, so don’t hesitate to prepare in advance.