Remove stems from spinach and tear into bite-size pieces. Wash spinach 2 or 3 times in cold water to remove all dirt and grit. Drain well or spin-dry in salad spinner to remove as much water as possible. Place spinach in large bowl.
Separate onion slices into rings, thinly slice water chestnuts and eggs, and crumble bacon. Add sliced onions, eggs and crumbled bacon to spinach.
Toss to combine ingredients. If not serving immediately, cover tightly with plastic wrap, and refrigerate until ready to serve.
Combine remaining ingredients in a small jar; shake until well blended.
When ready to serve, shake dressing again and pour over salad. Toss lightly until well mixed.