If you don't have any cooking spray, you can also simply rub some butter inside the mug. You are using a larger mug because the eggs will expand while being cooked. Keep beating them until the yolks are all broken and mixed in with the whites. There should be no streaks or swirls. Your eggs likely will not be set all the way. This is fine, because you still need to add some toppings and stir them up a bit. Some of the fillings, such as shredded cheese, can be used fresh, but others, such as sausage or bacon, need to be cooked first.  You can use things like diced bacon, chopped scallions, or shredded cheese. You can make the filling just one ingredient, or play around with different combinations. For more ideas on toppings, click here. How long you microwave the eggs for depends on the strength of your microwave. They are ready when they are puffy and all liquid has been absorbed. You can eat it straight out of the mug, or you can eat it on a plate. To get the omelet out, run a knife around the edges of the omelet in the mug, then dump it out onto a plate.

Summary:
Spray the inside of a 12 to 16-ounce (350 to 475 milliliters) microwave-safe mug with non-stick cooking spray. Add the eggs, salt, and pepper into the mug and beat them together with a fork. Microwave for 1 minute. Add any fillings, if desired. Give the eggs a quick stir and microwave them for another 1 to 2 minutes. Serve the omelet.