How to Make It
Preheat oven to 400°F with rack in highest position. Butter a 3-quart baking dish.
Bring a large pot of salted water to a boil over high. Add half of the asparagus and cook until crisp-tender, 2 to 4 minutes (depending on thickness of spears). Remove with tongs, letting excess water drip off, and transfer to prepared dish. Repeat with remaining asparagus; toss with 1 teaspoon of the salt and ¼ teaspoon of the black pepper.
Discard water; melt 2 tablespoons of the butter in same pot over medium-low. Whisk in flour and cayenne and cook, stirring constantly, until fragrant and slightly darker in color, about 2 minutes. Slowly whisk in milk. Increase heat to medium, and simmer, whisking often, until thickened, about 5 minutes. Slowly add Gruyère, cheddar, ½ teaspoon of the salt, and remaining ¼ teaspoon black pepper, and whisk until cheese is melted. Pour cheese sauce evenly over asparagus.
Melt remaining 2 tablespoons butter in a skillet over medium; stir in Panko and remaining ½ teaspoon salt. Scatter over asparagus.
Bake until warmed through, 8 to 10 minutes. Increase oven temperature to broil; broil until breadcrumbs are golden brown, 1 to 2 minutes.