Preheat the oven. Set your oven to 425°F (220°C). Line a large baking sheet with parchment paper or lightly spray with nonstick spray.

Prep the sprouts. Rinse, trim the stem ends, and remove any yellowed leaves. Slice each sprout in half through the core so they hold together.

Dry thoroughly. Pat the sprouts completely dry with paper towels. Dry sprouts crisp better and won’t steam on the pan.

Make the garlic-butter oil. In a large bowl, whisk together olive oil, melted butter, and minced garlic. Add salt, pepper, and optional red pepper flakes.

Toss to coat. Add the Brussels sprouts to the bowl and toss until every piece is evenly coated. Sprinkle half the Parmesan and toss again.

Arrange for crispiness. Spread the sprouts cut-side down on the baking sheet in a single layer with space between them. Overcrowding leads to steaming, not browning.

Roast. Bake for 18–22 minutes, until the cut sides are deeply golden and the centers are tender. Shake the pan once after 12 minutes for even browning.

Finish with cheese. Remove from the oven and immediately sprinkle the remaining Parmesan over the hot sprouts. Add lemon zest if using for a bright finish.

Taste and adjust. Season with a pinch more salt and pepper if needed. Garnish with chopped parsley and serve hot.