Preheat oven to 500 degrees F. Trim the bottom 2 inches off the broccolini stems and cut any thick stems in half.
Line a large jelly roll pan with foil (or use two baking sheets) and lightly spray with cooking spray. Add broccolini to pan and drizzle with olive oil and sprinkle with salt and pepper. Add garlic and toss until evenly coated. Line broccolini in a single layer.
Roast for 12 minutes then remove from oven and evenly space cubed butter on broccolini. Return to oven and bake an additional 3 minutes or until broccolini reaches desired crisp-tenderness. Broil if desired for a few minutes. Remove from oven and toss with lemon juice to taste. Season with additional salt and pepper to taste if desired.