Preheat oven to 400 degrees.
Cut off the brown ends of 1½ pound brussels sprouts and cut in half. Mix them in a bowl with the ¼ cup olive oil. Pour them onto a baking sheet and roast for 25-30 minutes, until crisp on the outside and tender on the inside.
Meanwhile, bring ⅓ cup 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, 3–4 minutes.
Remove from heat and add ⅓ cup balsamic vinegar and Pinch crushed red pepper flakes, if using, and whisk until sauce is smooth. Return saucepan to medium heat, add 2 Tablespoons butter and ½ teaspoon kosher salt, and cook, whisking constantly, until the glaze is slightly thickened, 3–4 minutes.
Drizzle the balsamic reduction over the roasted sprouts. Sprinkle with the 1 teaspoon lemon zest and serve immediately.
