In a small bowl, whisk together the vinegar, mustard and lemon-pepper. Slowly drizzle in the olive oil, whisking continuously. Set aside.
In a large bowl, mix together the spinach, mushrooms and onion. Toss with the dressing. Add in the shredded chicken.
Microwave the bacon following package directions, about 30 seconds. Crumble and sprinkle over salad. Serve immediately.