1. Spray the inside of a 12-ounce microwave-safe mug with cooking spray.
2. Combine eggs, milk, salt and pepper in the mug with a fork and stir well. Stir in vegetables and cheese.
3. Microwave on HIGH for 45 seconds and then stir. Return to the microwave and cook on HIGH unti the mixture has puffed and set, 60 to 90 seconds. The omelet may look wet on the top but it will dry as it cools.