Bring a large pot of salted water to a rolling boil. Add the trimmed asparagus pieces and blanch for 2-3 minutes, until bright green and tender-crisp. Immediately drain and plunge into an ice bath to stop cooking. Once cooled, drain thoroughly and set aside.

In a medium saucepan, melt 4 tablespoons of butter over medium heat. Whisk in the flour and cook for 1-2 minutes, stirring constantly, to create a roux.

Gradually whisk in the milk and chicken broth until smooth, ensuring no lumps. Bring the mixture to a gentle simmer, stirring frequently, until the sauce thickens (about 5-7 minutes).

Remove the saucepan from the heat. Stir in the garlic powder, onion powder, nutmeg, salt, and black pepper. Add the shredded Gruyere and sharp cheddar cheeses, stirring until completely melted and smooth.

Preheat oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish.

Add the blanched and drained asparagus to the cheese sauce and gently stir to combine. Pour the mixture into the prepared baking dish, spreading evenly.

In a small bowl, combine the Panko breadcrumbs and grated Parmesan cheese. Drizzle with 2 tablespoons of melted butter and toss gently to coat.

Sprinkle the Panko-Parmesan topping evenly over the asparagus mixture in the baking dish.

Bake for 20-25 minutes, or until the casserole is bubbly around the edges and the topping is golden brown and crispy.

Remove from oven and let stand for 5 minutes before serving.