Preheat your oven to 425°F (220°C). Line a large, heavy-duty baking sheet with parchment paper or aluminum foil. This lining step is crucial to prevent the Parmesan from sticking and burning directly onto the pan.

In a very large mixing bowl, combine the trimmed and halved Brussels sprouts and the 3 tablespoons of olive oil. Toss thoroughly until every piece is lightly coated with oil.

Season the sprouts by adding the garlic powder, kosher salt, black pepper, and red pepper flakes (if using) to the bowl. Toss again until the seasonings are evenly distributed across the sprouts.

Sprinkle the finely grated Parmesan cheese over the seasoned sprouts. Toss gently one last time, ensuring the cheese adheres to the oil on the surface of the sprouts. Do not overmix; you want the cheese primarily on the outside surface.

Arrange the Brussels sprouts onto the prepared baking sheet in a single layer. It is critically important that the sprouts do not overlap or crowd the pan, as this will steam them instead of roasting them. If necessary, use two baking sheets.

Maximize Crust: As you arrange them, ensure that as many sprouts as possible are placed with their flat, cut sides facing directly down onto the baking sheet. This maximizes the surface area contact, leading to a crispier, golden Parmesan crust.

Initial Roast: Place the baking sheet in the preheated oven and roast for 15 minutes.

Flip and Finish: After 15 minutes, remove the pan. Using a sturdy metal spatula, gently flip the sprouts. Return the pan to the oven and roast for an additional 8 to 12 minutes, or until the sprouts are tender and the Parmesan crust on the bottom is deeply golden brown and crunchy.

Remove from the oven and serve immediately.