Trim spinach of thick stems and wilted or yellowed leaves and wash thoroughly. Pat dry with paper towels or dry in a salad spinner.

Fry bacon in a heavy skillet over medium heat until golden brown and crisp, about 15 minutes. Remove bacon from skillet and drain on paper towels.

To make the dressing, strain bacon fat through a sieve, wipe out skillet with a paper towel, and return fat to skillet. Add brown sugar to fat, stir over low heat until dissolved, then add red wine vinegar to taste, up to 1 tbsp.

Put spinach, mushrooms, and eggs in a salad bowl. Crumble bacon on top and then pour warm dressing over the salad. (Dressing should be not quite hot enough to wilt spinach.) Toss a few times to evenly coat spinach leaves. Season to taste with freshly ground black pepper.