Place eggs in a saucepan and cover completely with cold water. Bring water to a boil. Cover, remove from heat, and let eggs stand in hot water for 10 to 12 minutes. Remove from hot water, cool, peel and chop.
In a small saucepan, combine the diced onion, sugar, black pepper, celery salt, mustard, vegetable oil, and vinegar. Whisk over high heat until all ingredients are well mixed and onions are translucent. Remove from heat and refrigerate until cool.
In a large bowl, combine the eggs, spinach, iceberg, bacon, croutons and onion rings. Toss together.