Place the eggs in a pot covered with cold water. When the water comes to a boil, turn off the heat and let them sit in the hot water for 20 minutes. Remove the eggs from the hot water and immediately cool with either ice or cold water. Peel and slice the eggs. Set aside.
Cook the bacon and drain on paper towel. Crumble the bacon and set aside. Reserve 3 tablespoons of the bacon grease.
Wash the baby spinach and remove any large stems. Drain and pat dry thoroughly. Place in a large mixing bowl. Slice the red onion very thin add it to the spinach. Clean the mushrooms and slice. Add the mushrooms to the spinach. Add the sliced eggs and crumbled bacon.
In the same frying pan that you cooked the bacon, add the reserved bacon grease. Over low heat, whisk in the red wine vinegar, sugar and Dijon mustard. Season with salt and pepper. Drizzle the dressing over the spinach and toss to combine. Sprinkle the crumbled blue cheese over the top. Serve immediately.