Add mushrooms and onions to skillet; cook and stir on medium heat 5 minutes or until vegetables are crisp-tender.
Beat eggs and milk with wire whisk until well blended. Add to vegetable mixture; stir until well blended. Cook 2 minutes or until egg mixture begins to set but top is still moist, stirring occasionally.
Stir in Canadian bacon; continue cooking 2 minutes or until egg mixture is set and no visible liquid remains, stirring occasionally.
Season with pepper to taste; sprinkle with cheese.
Place 1/2 cup of the spinach on each of 4 salad plates; top evenly with the egg mixture. Sprinkle with tomatoes.