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 thinly sliced garlic cloves, 3 tablespoons olive oil, ½ teaspoon kosher salt, and ¼ teaspoon red pepper flakes together in a large bowl.
Remove the hot baking sheet from the oven. Transfer the broccoli to the baking sheet 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, 15 to 17 minutes. Sprinkle with ½ ounce finely grated Parmesan cheese, 1 teaspoon finely grated lemon zest, and 1 tablespoon lemon juice. Toss to combine.
