1. In a microwave oven, place the box of spinach with the outer wrapper removed and cook on HIGH (9) until it starts separating. Remove it from oven.
2. Place a glass casserole and add tuna, spinach, mushrooms, soup, pepper and lemon juice, combined well.
3. Cover and cook on a setting of REHEAT (8) for another 8 minutes until the mixture turns hot.
4. Top with sliced tomato and cheddar cheese, cover and continue cooking on a setting of REHEAT (8) for another 3 minutes until the cheese softens.
5. Serve the Tuna spinach casserole after garnishing with onion rings.