Step 1

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

Step 2

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

Step 3

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), about 3 minutes. Remove from heat; add vinegar and red pepper flakes, if using, and whisk until sauce is smooth (it will bubble quite aggressively when you first add the vinegar). Set saucepan over medium heat, add butter and remaining 1/2 tsp. salt, and cook, whisking constantly, until glaze is glossy, bubbling, and slightly thickened, about 4 minutes.

Step 4

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