Combine 6 cups torn spinach, 1 cup sliced mushrooms, and 1/2 cup thinly sliced red onion in a large bowl. Combine 1/3 cup fat-free ranch dressing*, 1/4 cup (1 ounce) grated fresh Parmesan cheese, if desired, and 1/2 teaspoon freshly ground black pepper in a small bowl; spoon over salad. Toss gently to coat.
*Check for gluten
Cooking Light Gluten-Free Cookbook