Preheat oven to 425°. Cut broccoli florets in half through the stem so they'll lie flat while roasting for nice, even browning. Divide broccoli between 2 large rimmed baking sheets.
Drizzle 1/4 cup oil over broccoli, sprinkle with salt and pepper, and turn until well coated.
Roast, switching pan positions and turning broccoli with tongs halfway through cooking, until tender-crisp and beginning to brown on the bottom, 10 to 15 minutes. Drizzle with oil.
To serve, spread a layer of pesto on a platter. In a bowl, gently toss broccoli with more pesto and arrange on platter.