Place a rimmed baking sheet on the bottom rack of your oven; heat oven to 450 F.

Toss Brussels sprouts and oil in a large bowl; season with salt and black pepper.

Carefully remove baking sheet from oven. Using tongs, arrange Brussels sprouts cut-side down on the baking sheet. Roast 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 (it will be foamy; that’s okay), 3-4 minutes.

Remove from heat; add vinegar and red pepper flakes, if using, and whisk until sauce is smooth (it will bubble up quite aggressively when you add the vinegar before settling).

Return saucepan to medium heat, add butter and ½ teaspoon salt; cook, whisking constantly, until glaze is glossy, bubbling and slightly thickened, 3-4 minutes.

Transfer Brussels sprouts to a large bowl. Add glaze and scallions and toss to combine. Transfer to a platter and top with lemon zest.