01 - Preheat the oven to 425°F. Line a baking sheet with parchment paper.
02 - Arrange the asparagus spears in a single layer on the prepared baking sheet. Drizzle with olive oil and sprinkle with kosher salt and black pepper. Toss to coat evenly.
03 - Roast the asparagus for 12 to 15 minutes, or until tender and lightly browned, shaking the pan halfway through cooking.
04 - In a heatproof bowl set over a saucepan of barely simmering water, whisk together the egg yolks and lemon juice constantly until slightly thickened.
05 - Gradually whisk in the melted butter, a little at a time, until the sauce is thick and glossy. Stir in Dijon mustard if using, cayenne pepper, and salt to taste. Remove from heat immediately.
06 - Arrange the roasted asparagus on a serving platter and spoon the Hollandaise sauce generously over the top. Serve immediately.