Preheat the oven to 375°F. Lightly grease a 9×13-inch baking dish.

In a mixing bowl, combine the garlic herb cream cheese and 1/2 cup of grated white cheddar. Mix well. Stuff each mushroom cap generously with this mixture.

To make the alfredo sauce, melt the butter in a saucepan over medium heat. Add the garlic and sauté for 1 minute. Whisk in the flour and cook for another minute. Gradually add the heavy cream and half-and-half, whisking constantly. Add the Parmesan, white cheddar, and Gruyère, stirring until melted and smooth. Season with salt and white pepper.

Pour about half of the alfredo sauce into the prepared baking dish, just enough to cover the bottom of the dish. Nestle the stuffed mushroom caps (filling side up) into the sauce and spoon a light layer of sauce over the top of each mushroom. Cover all mushrooms with the remaining 1 cup of white cheddar cheese.

Combine all the ingredients for the buttered breadcrumb topping in a small bowl. Mix well. Sprinkle the breadcrumbs over the mushrooms.

Bake for 10 to 12 minutes, until the mushrooms are tender and the cheese is bubbly.

Broil on low for 2 to 3 minutes, until the breadcrumbs are golden brown. Watch closely to prevent over-browning.

Garnish with minced parsley and serve immediately from the pan.