1. Preheat the oven to 425°F (220°C) and place a rimmed baking sheet on the middle rack while heating. Alternatively, line the baking sheet with parchment paper for easier cleanup.
2. Trim off dry or woody stems from the Brussels sprouts and remove loose outer leaves. Slice larger sprouts in half or quarters to maintain even size for uniform cooking; smaller sprouts can be left whole.
3. Toss the Brussels sprouts thoroughly with olive oil, salt, and pepper in a large bowl.
4. Arrange the sprouts cut side down in a single layer on the hot baking sheet. This promotes faster, even browning and crispiness.
5. Roast for 20 to 25 minutes (up to 30 minutes depending on size), until the sprouts are tender and golden brown around the edges. Check periodically to avoid burning.
6. Optional: After roasting, drizzle with lemon juice, sprinkle with lemon zest, Parmesan cheese, thyme leaves, and red pepper flakes, then toss gently. Alternatively, serve with lemon wedges or drizzle with maple syrup before letting them sit in the turned-off oven for 2 minutes to absorb flavors. Garnish with parsley if desired.