Remove the stems from the spinach and wash, drain and pat dry thoroughly. Place into a large mixing bowl and refrigerate until ready to use.
Prepare two hard boiled eggs, peel and slice each egg into 8 pieces and set aside. While the eggs are cooking, fry the bacon and remove to a paper towel to drain, reserving 3 tablespoons of the rendered bacon fat.
Transfer the fat to a small pan set over low heat and whisk in the red wine vinegar, sugar and Dijon mustard. Season with a small pinch each of kosher salt and black pepper.
Add the mushrooms and the sliced onion to the spinach and toss. Add the dressing and bacon and toss to combine. Divide the spinach between 4 plates and garnish with the hard boiled eggs. Serve immediately.