Simmer water and onion in a small saucepan, uncovered, until onion is softened and most of water is evaporated, 10 to 15 minutes. Purée in a blender with vinegar, caraway seeds, sugar, mustard, and 1/2 teaspoon salt. With motor running, add oil in a slow stream and blend until dressing is emulsified.
Combine lettuce, celery, and celery leaves in a large bowl and toss with just enough dressing to coat.
Adapted from epicurious.com