Arrange a rack in the middle of the oven. Place a rimmed baking sheet on the rack and heat the oven to 425°F.
Toss 1 pound broccoli florets, 2 tablespoons olive oil, and ¼ teaspoon black pepper together in a large bowl.
Remove the hot baking sheet from the oven. Add the broccoli and arrange into a single layer cut-side down. Return the baking sheet to the oven and roast until the broccoli is crisp-tender and browned, 13 to 15 minutes.
Meanwhile, stir and smash 2 tablespoons room-temperature unsalted butter, 2 teaspoons white miso paste, 1 teaspoon finely grated lemon zest, 1 teaspoon lemon juice, and ⅛ teaspoon red pepper flakes if using together in a large bowl.
Transfer the roasted broccoli to the bowl and toss until the butter is melted.
