Bring a large pot of salted water to a boil. Add the 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. Drain again thoroughly and set aside.

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

In a large skillet over medium heat, melt 2 tablespoons of butter. Add the chopped yellow onion and cook for 3-5 minutes until softened and translucent. Add the minced garlic and cook for another minute until fragrant.

Sprinkle the flour over the onion and garlic. Cook, stirring constantly, for 1-2 minutes to create a roux.

Gradually whisk in the milk and broth, a little at a time, until smooth and no lumps remain. Continue to cook, stirring frequently, until the sauce thickens to a creamy consistency (about 5-7 minutes).

Remove the skillet from heat. Stir in the Dijon mustard, salt, and black pepper. Add the 1/4 cup grated Parmesan cheese and 1/2 cup shredded Gruyere cheese, stirring until melted and smooth.

Gently fold the blanched asparagus into the creamy cheese sauce. Pour the mixture into the prepared baking dish and spread evenly.

In a medium bowl, combine the panko breadcrumbs, 1/4 cup grated Parmesan cheese, melted butter, garlic powder, and a pinch of black pepper. Mix well until the breadcrumbs are evenly coated.

Sprinkle the cheesy crumb topping evenly over the asparagus mixture in the baking dish.

Bake for 25-30 minutes, or until the casserole is bubbling around the edges and the crumb topping is golden brown and crispy.

Remove from oven and let stand for 5-10 minutes before serving.