Prepare the Oven and Baking Surface: Set your oven to preheat at 425°F (220°C). Line a baking sheet with parchment paper to prevent sticking and simplify cleanup.
Trim and Halve the Brussels Sprouts: Wash the Brussels sprouts, remove any damaged outer leaves, and trim the tough ends. Slice each sprout in half lengthwise to ensure they cook evenly.
Season and Arrange the Sprouts: Place the halved sprouts in a large bowl. Drizzle with olive oil and sprinkle generously with sea salt and freshly ground black pepper. Toss well to coat each piece. Spread the sprouts in a single layer on the prepared baking sheet, leaving space between them to promote crispiness.
Roast Until Golden and Tender: Roast in the preheated oven for about 20 to 30 minutes. Stir or shake the pan halfway through to encourage even browning. The Brussels sprouts are ready when they develop a deep golden color around the edges and feel tender when pierced with a fork.
Toss with Bright Lemon and Parmesan: Once roasted, transfer the sprouts to a serving bowl. Add the fresh lemon juice, lemon zest, grated Parmesan, and thyme leaves. Gently toss to combine and coat the sprouts with the fresh flavors.
Garnish and Serve: Sprinkle with fresh parsley leaves and, if you like a little heat, a pinch of red pepper flakes. Serve warm as a flavorful side dish.