Spray a baking sheet with olive oil non-stick cooking spray. Line baking sheet with parchment paper, and then spray paper with cooking spray. Place 1/2 cup shredded cheese on half of the baking sheet and shape into circle about 5 inches in diameter. Form a second circle on other half of baking sheet. (Try using a frozen whipped topping plastic top as a pattern for shaping the cheese crisps.)
Bake for about 5 minutes, or until cheese is golden around edges. Using a wide spatula, carefully turn cheese rounds over. Bake for about 2 minutes longer, or until deep golden color around edges and set in center. Using the spatula, carefully remove rounds and drape them over a large rolling pin suspended like a bridge between 2 tall cans to shape. Let cool while shaped over rolling pin. Repeat with remaining cheese to make 2 more circles.
In a medium bowl, combine mushrooms, tomatoes, spinach, and onion. Add dressing and toss to combine. To serve, place a cheese crisp on each of 4 salad plates and then top with a spoonful of salad.