Add spinach, mushrooms, and tomatoes to a mason jar. Cover and refrigerate until ready to use.
When ready to make the omelet, add the eggs. Shake until eggs are broken and scrambled.
Pour jar contents into OXO's Microwave Omelet Maker. Without closing the omelet maker, microwave for 3 minutes or until the center is no longer runny. Fold over the omelet maker. Transfer omelet to a plate and serve warm.