In a large sauté pan, heat 1 tablespoon olive oil on medium heat. Add mushrooms, salt and pepper, and stir. Cook until mushrooms are tender and browned, about 2-3 minutes.
Add spinach and garlic. Cook, stirring often to prevent burning, until spinach releases water and it evaporates, about 2-3 minutes. Place cooked vegetables in a bowl and set aside.
Starting with a clean sauté pan, heat remaining olive oil on medium heat, tilting pan to coat evenly. When oil is hot, but not smoking, add eggs.
As eggs cook, use a rubber spatula to push the edges of the egg mixture toward center of pan, allowing uncooked eggs to flow underneath and make contact with the pan. Repeat procedure until all eggs are set.
When eggs are set, but still shiny, top with vegetable mixture and shredded cheese. Remove from heat, fold omelet in half and slide onto a plate to serve.
Note: prepare in 2 batches for easier handling.