1: Place a rimmed baking sheet on the bottom rack of the oven; preheat to 450°.
2: Toss brussels sprouts and oil in a large bowl; season with salt and black pepper.
3: Carefully remove baking sheet from oven. Using tongs, arrange brussels cut side down on baking sheet.
4: Roast brussels on bottom rack until softened and deeply browned, 20–25 minutes.
5: Meanwhile, bring honey to a simmer in a small saucepan over medium-high heat.
6: Reduce heat to medium-low and cook, stirring often, until honey is a deep amber color but not burnt, 3–4 minutes.
7: Remove from heat and add vinegar and red pepper flakes, if using, and whisk until sauce is smooth.
8: 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.
9: Transfer brussels sprouts to a large bowl. Add glaze and scallions and toss to combine.
10: Transfer to a platter and top with lemon zest.
