1. Preheat oven to 400 degrees Fahrenheit. Line and spray a baking sheet with nonstick cooking spray.
2. Place baby bella mushrooms and onions on baking sheet and spray with nonstick cooking spray. Season with salt and pepper, and bake for 10 minutes. Let cool.
3. Prepare the dressing. In the bowl of your food processor or blender, combine 1 cup of mushrooms and onions with light sour cream, chives, olive oil, salt and pepper. Reserve remaining mushrooms and onions.
4. In a large bowl, combine spinach leaves, mushrooms, onions, tomatoes, and red onion. Toss with dressing and serve.