A basic vinaigrette would overpower delicate red-leaf lettuce; instead, we opted for a subtle dressing that gets its creaminess from puréed onions.
Cooks' notes:
· Dressing can be made 3 days ahead and chilled in an airtight container. · Lettuce can be washed and dried and celery can be cut 1 day ahead and chilled in sealed bags lined with paper towels.