1. Preheat half the butter in a pan over medium heat.
2. Once the butter and pan are hot, add in the mushrooms to cook for 2 to 4 minutes.
3. While the mushrooms cook, whisk together the eggs, onions, garlic, pepper, and salt, and set aside.
4. Once the mushrooms are cooked, remove them from the pan, set them aside, and return the pan to the heat.
5. Once the pan is hot again, add in the second half of the butter.
6. Once the butter is melted, pour the whisked egg mixture into the pan to cook for 3 to 5 minutes, covered.
7. Once the omelette has mostly set, dump the mushrooms into the center of the omelette, and then fold the sides over the mushrooms (almost like wrapping them into an egg burrito), and let it cook for another 1 to 2 minutes.
8. After that time, remove the omelette from the heat, serve it up, and dig in!