Prepare the Oven and Baking Surface: Preheat your oven to 425°F (220°C). Line a baking sheet with parchment paper to ensure the Brussels sprouts roast evenly without sticking and to make cleanup effortless.
Trim and Slice Brussels Sprouts: Rinse the Brussels sprouts under cold water, then trim off the tough stem ends. Slice each sprout in half lengthwise to help them cook evenly and develop a crispy, caramelized exterior.
Season and Oil the Sprouts: Place the halved sprouts in a large mixing bowl. Drizzle with the olive oil, then sprinkle with sea salt and freshly ground black pepper. Toss everything gently but thoroughly, making sure each piece is coated in oil and seasoning.
Roast Until Golden and Tender: Arrange the Brussels sprouts in a single layer on the prepared baking sheet, cut side down for maximum browning. Place in the oven and roast for 20 to 30 minutes, turning once halfway through. Roast until they are tender when pierced with a fork and the edges are beautifully golden and crisp.
Add Fresh Finishing Touches: Once roasted, transfer the sprouts back to the mixing bowl. Add the fresh lemon juice, lemon zest, Parmesan cheese, and thyme leaves. Toss gently to combine so the flavors meld without crushing the sprouts.
Garnish and Serve: Transfer the Brussels sprouts to a serving dish. Sprinkle fresh parsley leaves on top and add a pinch of red pepper flakes if desired, for a little heat. Serve immediately and enjoy the bright, savory flavors.