Make the Dressing: Whisk together the dressing ingredients in a medium saucepan and cook over low heat for 1 hour, stirring occasionally. Let cool slightly and re-whisk before serving.
Arrange the spinach on plates and top with an equal amount of bacon, mushrooms, onion and sliced eggs. Sprinkle with salt and pepper, drizzle the warm dressing over top and serve. Any leftover dressing can be stored in an airtight container in the refrigerator.