1. Preheat oven to 375°F. Grease 10-inch oven-safe skillet or 9-inch pie dish with butter or cooking spray.

2. Heat butter or olive oil in medium skillet over medium-high heat. Add diced onion and cook 3-4 minutes until softened and translucent.

3. Add sliced mushrooms and minced garlic to skillet. Cook 6-8 minutes, stirring occasionally, until mushrooms release moisture and become golden brown. Season with pinch of salt and pepper.

4. In large bowl, whisk together eggs, milk, remaining salt, pepper, thyme, and nutmeg if using. Whisk until mixture is well combined and slightly frothy.

5. Stir in three-quarters of shredded cheese, reserving rest for topping. Add chopped parsley to egg mixture.

6. Spread cooked mushroom mixture evenly in prepared skillet or pie dish. Pour egg mixture over mushrooms, using fork to distribute vegetables evenly.

7. Sprinkle remaining cheese over top of frittata. This creates beautiful golden crust during baking.

8. Bake 25-30 minutes until eggs are completely set and top is golden brown. Frittata should puff slightly and knife inserted in center should come out clean.