1. Preheat the Oven: Preheat your oven to 375°F (190°C). Grease a 9-inch pie dish or baking dish with butter to prevent sticking.
2. Prepare the Egg Mixture: In a large mixing bowl, whisk together the eggs, milk (or half-and-half), sour cream (if using), salt, and black pepper until smooth and well combined.
3. Add the Fillings: Stir in the diced ham, diced onion, shredded cheddar cheese, and grated Parmesan cheese until evenly distributed.
4. Pour and Bake: Pour the mixture into the greased baking dish and spread it evenly. Bake for 25–30 minutes, or until the quiche is golden brown and the center is set.
5. Let Cool & Serve: Allow the quiche to cool for a few minutes before slicing. Serve warm and enjoy!