Preheat the Oven:
Preheat your oven to 350°F (175°C) and lightly grease a 9-inch pie dish or quiche dish with butter or cooking spray.

Sauté the Onions:
In a medium skillet, heat 1 tablespoon of olive oil or butter over medium heat. Add the diced onion and sauté until softened and translucent, about 4-5 minutes. Remove from heat and set aside to cool slightly.

Whisk the Eggs and Milk:
In a large bowl, whisk together the 6 eggs, 1 cup of milk (or half-and-half if you want a creamier quiche), and sour cream (if using). Season with salt and pepper to taste. The sour cream will give the quiche a richer, creamier texture, but you can leave it out if you prefer a lighter version.

Add the Cheese and Ham:
Stir in the shredded cheddar cheese, grated Parmesan cheese, and diced ham. Add the sautéed onions to the mixture and mix everything until well combined.

Pour the Mixture into the Baking Dish:
Pour the egg, cheese, ham, and onion mixture into the prepared pie dish or quiche dish. Spread it out evenly with a spatula.

Bake the Quiche:
Place the dish in the preheated oven and bake for 35-40 minutes, or until the quiche is set in the center and golden brown around the edges. You can check for doneness by inserting a knife or toothpick into the center—if it comes out clean, the quiche is ready.

Cool and Serve:
Allow the quiche to cool for about 5 minutes before slicing and serving. This will help it set further and make slicing easier.