Step 1: Preheat your oven to 400°F (200°C). Line a large baking sheet with parchment paper or aluminum foil for easier cleanup, if desired.
Step 2: Prepare the broccolini. Wash the broccolini thoroughly under cold water. Trim about an inch off the tough, dry ends of the stalks. If any stalks are particularly thick, you can slice them lengthwise to ensure even cooking. Pat the broccolini completely dry with paper towels.
Step 3: In a large mixing bowl, combine the dried broccolini, olive oil, minced garlic, sea salt, and black pepper. Toss well to ensure all the broccolini is evenly coated with the oil and seasonings.
Step 4: Arrange the seasoned broccolini in a single layer on the prepared baking sheet. Ensure there’s enough space between the stalks for air to circulate. If your baking sheet is too crowded, the broccolini will steam instead of roast, so use two sheets if necessary.
Step 5: Roast for 10-12 minutes. At this point, carefully remove the baking sheet from the oven and sprinkle the grated Parmesan cheese evenly over the broccolini. Return to the oven and roast for another 3-5 minutes, or until the broccolini is tender-crisp, slightly charred at the edges, and the Parmesan is melted and golden brown.
Step 6: Once done, remove from the oven. If desired, squeeze fresh lemon juice over the roasted broccolini for a bright finish, and sprinkle with a pinch of red pepper flakes for a subtle heat. Serve immediately.