Fry onions with olive oil until golden brown.
Add chopped button mushrooms, black pepper and garlic powder. Let them cook for 3 - 5 minutes. Set them aside.
In the same pan, melt butter then add a mixture of eggs, milk, butter and salt. As it cooks, add shredded cheddar cheese and half of the cooked mushrooms.
Cover the pan and let it cook on low heat until done to your preference.
Take the omelette off the heat and add the remaining mushrooms, cheese and garnish with parsley.