Wash and dry 8 cups of spinach thoroughly, as any excess moisture can dilute the dressing.
Hard-boil 2 large eggs, peel, and slice them into quarters.
Cook 4 slices of bacon until crisp. Crumble the bacon once cooled. Reserve 4 tablespoons of the bacon grease.
Mix 4 tablespoons bacon grease, 3 tablespoons red wine vinegar, 1/2 teaspoon Dijon mustard, and 1 tablespoon sugar in a small bowl. Stir until sugar dissolves.
In a large salad bowl, toss the spinach with sliced mushrooms and red onions, then add the eggs and crumbled bacon.
Pour the warm dressing over the salad, then add salt and pepper to taste. Toss until well coated.