Cut broccoli into 3" lengths; cut thick pieces in half lengthwise. Place in a roasting pan, and drizzle with olive oil; toss to coat well. Sprinkle with salt and pepper. Roast at 500° for 14 minutes, stirring once after 8 minutes.
Meanwhile, toast walnuts in melted butter in a skillet over medium-high heat 5 minutes or until fragrant.
Spoon broccoli into a serving bowl. Squeeze cut lemon over broccoli, tossing well. Stir in walnuts and butter.