Wash the asparagus thoroughly under cold water and pat dry completely—this helps them brown better in the oven. Snap off the woody ends by bending each spear gently until it breaks naturally at the tender part, or trim them with a knife if you prefer. While the asparagus dries, preheat your oven to 400°F. In a large baking dish or on a rimmed sheet pan, combine the olive oil, salt, pepper, garlic powder, and red pepper flakes. Toss the trimmed asparagus in this seasoned oil until evenly coated, then arrange them in a single layer.

Distribute the butter in small pieces evenly across the asparagus, then scatter the lemon slices over the top. The butter will melt in the oven and create a rich sauce with the lemon. Roast at 400°F for 10 minutes until the asparagus is tender-crisp and the lemon begins to caramelize slightly at the edges. I like to let the butter brown just a touch—it adds a subtle nuttiness that really elevates the dish.

Remove the pan from the oven and immediately sprinkle the shredded parmesan evenly over the hot asparagus. The residual heat will help it meld with the butter and lemon sauce. Switch your oven to broil and return the pan to the top rack for 2-3 minutes, watching carefully to prevent the cheese from burning. You want a light golden color on the cheese with just a hint of char on the lemon slices for the best flavor.