In a mixing bowl, combine the vinegar, salt, sugar, mustard powder, pickle relish, and capers. Add the vegetable oil and olive oil and mix well. Store the dressing in an air-tight container in the refrigerator.
Grate the eggs on a medium-sized grater. Cover and set aside.
Wash the spinach and remove any tough stems. Drain and pat dry with paper towels. Then transfer to a large bowl.
Sauté the bacon until crisp. Remove the bacon from the pan and sprinkle it over the spinach.
Add the onions and mushrooms to the skillet and sauté in the remaining bacon drippings until tender. Add 1 cup of the prepared dressing to the skillet and bring to a boil.
Pour the contents of the skillet over the spinach and toss. Garnish with the grated eggs and serve with additional dressing.
Recipe Tips
I like my eggs left in whole slices for more flavor. You may wish to grate your hard boiled eggs as directed in this recipe.