Position rack in center of oven and preheat the oven to 375 degrees F.
Line a baking sheet with parchment paper and arrange the bacon strips on it. Bake until brown and crispy, about 20 to 25 minutes.
Using oven mitts or pot holders, remove the bacon from the oven and let cool, about 15 minutes. Crumble into bite-size pieces.
Combine the spinach, bacon, mushrooms, and onion
in a large mixing bowl and toss.
Divide the spinach mixture among 4 large plates or 8 small plates (either 2-cup or 1-cup portions). Garnish with tomato wedges and or sliced hard-boiled eggs if desired. Drizzle with the dressing and serve.