Preheat your oven to 425°F (220°C). Line a large, heavy-duty baking sheet with parchment paper or aluminum foil. This 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 using your hands or a large spoon until every piece is lightly coated with oil.

Add the garlic powder, kosher salt, and 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 at this stage; you want the cheese to sit on the surface.

Spread 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.

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.

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

After 15 minutes, remove the pan from the oven. Using a sturdy metal spatula, gently flip the sprouts to expose new surfaces to the heat. Return the pan to the oven and roast for an additional 8 to 12 minutes, or until the sprouts are tender in the center, the outer leaves are nicely charred, and the Parmesan crust on the bottom is deeply golden brown and crunchy.

Remove from the oven and serve immediately.