In a mixing bowl, toss together the spinach, sliced onions, sauteed mushrooms, and boiled eggs.
Add the chopped bacon to a non-stick pan and place it over medium heat. Fry the bacon for 2-3 minutes, until crispy. Remove the bacon and add it to the salad.
Pour the bacon grease (rendering) into a small bowl, then whisk in the red wine vinegar and Dijon mustard. Toss it through the salad and serve immediately.