½ cup
grated Parmesan cheese, plus more for serving
¾ cup
grated Gruyère
¾ cup
Panko breadcrumbs
1 tablespoon
minced flat leaf parsley, (optional)
Method
Preheat the oven to 400°F with a rack in the center position.
Bring a large pot of salted water to a boil over high heat. Add the asparagus and cook until bright green and tender, about 4 minutes. Drain, pat dry and transfer to a baking dish.
In a small saucepan or skillet, melt 2 tablespoons of the butter over medium heat until browned and foaming. Whisk in the flour until thickened and fragrant, about 1 minute. Slowly add the milk to the butter mixture and cook, whisking, for 2 minutes, or until slightly thickened. Whisk in the mustard, salt, pepper, and nutmeg until fully incorporated. Add the parmesan and whisk to smooth.
Pour the sauce over the asparagus in the baking dish and sprinkle with the Gruyère. Bake until bubbly and beginning to brown, 15-20 minutes.
Meanwhile, melt the remaining tablespoon of butter in a small skillet over medium heat. Add the panko and cook, stirring, until the panko is golden brown. Set aside.
When the gratin is done, sprinkle with the toasted panko, parsley (if using), and parmesan cheese. Serve warm.