Place a rimmed baking sheet on the bottom rack of the oven; preheat to 450°F. Toss Brussels sprouts and olive oil in a large bowl; season with salt and black pepper.

Carefully remove baking sheet from oven. Using tongs, arrange Brussels cut side down on baking sheet. Roast Brussels on bottom rack until softened and deeply browned, 20–25 minutes.

Meanwhile, bring honey to a simmer in a small saucepan over medium-high heat. Reduce heat to medium-low and cook, stirring often, until honey is a deep amber color but not burnt, 3–4 minutes.

Remove from heat and add vinegar and red pepper flakes, if using, and whisk until the sauce is smooth. Return saucepan to medium heat, add butter and ½ tsp. salt, and cook, whisking constantly, until glaze is glossy, bubbling, and slightly thickened, 3–4 minutes.

Transfer Brussels sprouts to a serving dish and drizzle with the honey glaze. Garnish with scallions and lemon zest before serving.