Whisk together the dressing ingredients until the sugar dissolves. Taste and season with additional salt, if necessary. Set aside until ready to use, or store in an airtight container in the refrigerator for up to 4 days.
In a large bowl or rectangular dish, layer half of the spinach, half of the red onion, half of the mushrooms, and half of the eggs. Repeat the layers once more. Drizzle with dressing and top with bacon just before serving.
Notes
When preparing the dressing, make sure that you finely grate the onion. A microplane (or citrus zester) works really well for this task, since it practically purees the onion into a fine paste when grated. This gives you that great onion flavor without any big chunks in the dressing.
In order to prevent the spinach from wilting or becoming soggy, wait to add the dressing just before serving.
Add the bacon on top just before serving so that it stays nice and crispy.
You might not need all of the dressing, so drizzle it on the salad slowly so that the leaves are damp, and then serve a pitcher of extra dressing on the side in case your guests need more.