Preheat the oven to 425°F/220°C, and line a sheet pan with parchment paper for easy clean up.
Wash and pat dry broccolini with paper towels. Slice the thick stems in half, leaving a bit of stem on each floret so they hold together once cooked. Place right on the lined sheet pan in a single layer. If you want to roast garlic, slice the top of a garlic bulb.
Drizzle the broccolini and garlic with olive oil, and season with salt and pepper. Toss the broccolini so it's all coated in oil and seasonings.
Roast for 20-25 minutes flipping halfway through. Keep an eye on the broccolini as all ovens are different, and start checking on it at 15 minutes.
When the broccolini is ready, remove the sheet pan from the oven. If the garlic is squeezable, it means that it's ready. If not, roast for 10 more minutes (to help it roast quicker, wrap in foil).
Serve. Serve with a squeeze of fresh lemon juice, or a sprinkle of freshly grated parmesan cheese, and roasted garlic on the side (I like to smash it, and add it all over the broccolini).