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, and cool. Once cool, peel and chop.
Prepare the dressing in the blender by combining the onion, sugar, salt, oil, vinegar, pepper, celery seed and Dijon mustard. Blend until smooth.
Place spinach in a bowl, top with eggs, bacon, croutons and mushrooms. Pour enough dressing over salad to lightly coat. Toss and serve.