Coat a 12-ounce microwave-safe coffee mug with cooking spray and add hash browns.
Microwave on high for 1 minute. Add the egg and water; beat until blended. Microwave on high for 30 seconds; stir. Microwave until the egg is almost set, 15 to 30 seconds longer.
Season with salt and pepper and top with salsa and cheese as desired. Enjoy!