Place baking sheet in oven and preheat at 450 degrees. Toss Brussels sprouts and oil, season with salt and pepper.
Remove baking sheet from oven, using tongs arrange brussels cut side down on baking sheet. Roast brussels on bottom rack 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 and 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 tsp. salt, and 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-Yummy!
