Place a rack in the center of the oven and preheat the oven to 425 degrees F. Coat a large rimmed baking sheet with nonstick spray or line with parchment paper for easy cleanup.
Trim off the end of the broccolini stems. If your broccoli stalks are thick, cut them in half lengthwise. You want all of the broccolini pieces to be roughly the same diameter. Place the broccolini in the center of the prepared baking sheet.
Place the garlic on top of the broccolini. Drizzle with the oil and sprinkle with salt and pepper (if using red pepper flakes, add them here).
Toss to very thoroughly coat the broccolini with the oil and distribute the garlic evenly. Make sure all of the tips of the florets are nicely coated with oil to help protect them from burning. If any seem dry, drizzle on a little extra oil as needed. Arrange the broccolini in a single layer. Flip any cuts sides so that the cut part of the stem is touching the pan. (If you are adding Parmesan, sprinkle it over the top.)
Roast the broccolini for 10 to 15 minutes, until it is crisp-tender and beginning to char. Halfway through the baking time, rotate the pan 180 degrees. No need to turn the broccolini over. Remove from the oven and sprinkle with a pinch of additional salt and a squeeze of fresh lemon juice if desired. Enjoy immediately.
Notes
TO STORE: Refrigerate broccolini in an airtight storage container for up to 4 days.
TO REHEAT: Gently rewarm leftovers on a baking sheet in the oven at 350 degrees F.
TO FREEZE: Freeze broccolini in an airtight freezer-safe storage container for up to 3 months. Let thaw overnight in the refrigerator before reheating.