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.
Place bacon in a large, deep skillet. Cook over medium high heat until evenly brown. Drain, crumble and set aside.
Prepare the dressing in a blender by combining the onion, sugar, salt, oil, vinegar, pepper, celery seed and Dijon mustard. Blend until smooth.
In a large salad bowl, combine the eggs, bacon, spinach, croutons and mushrooms. Toss to mix. Pour enough dressing over salad to lightly coat. Toss and serve.
