Preheat your oven to 425°F and line a large baking sheet with parchment paper. While the oven heats, rinse the asparagus bunch thoroughly under cold water and pat completely dry with paper towels—this is crucial for achieving crispy, caramelized edges rather than steamed asparagus. Trim the tough, woody bottom 1-2 inches from each spear by either snapping them naturally where they break or using a knife to cut uniformly.

Arrange the trimmed asparagus spears in a single layer on your prepared baking sheet. Drizzle with the olive oil and sprinkle evenly with salt, black pepper, garlic powder, and red pepper flakes. Toss gently with your hands or two spoons to coat all the spears thoroughly. Arrange them so they're in a relatively even layer—I like to spread them out so they're slightly overlapping rather than completely spread apart, which helps them roast more evenly. Slice your lemon into thin rounds and scatter them among the asparagus.

Roast in the preheated 425°F oven for 10-15 minutes, until the asparagus is tender-crisp and the tips are lightly browned and caramelized. The lemon rounds will soften and release their juice directly onto the asparagus, infusing it with bright flavor. Remove from the oven and serve immediately while hot—the roasted lemon rounds are edible and add a wonderful tangy sweetness to the dish.