Step 1

Place racks in upper and lower thirds of oven and set a rimmed baking sheet on each; preheat oven to 500°. Toss brussels sprouts with oil in a large bowl to coat; season with kosher salt and pepper. Carefully (baking sheets will be hot!) divide brussels sprouts between baking sheets and spread out in a single layer; reserve bowl. Roast, rotating baking sheets top to bottom and front to back halfway through, until brussels sprouts are charred in spots and tender, 16–18 minutes.

Step 2

Meanwhile, if using walnuts, toast in a dry medium skillet over medium heat, tossing often, until fragrant and slightly darkened, about 4 minutes. Transfer to a small bowl and let cool.

Step 3

Cook butter in a small saucepan over medium-low heat, swirling occasionally, until milk solids are a deep amber color and butter smells very nutty, 5–8 minutes. Remove from heat and stir in gochujang and maple syrup. Season with kosher salt and pepper.

Step 4

Combine brussels sprouts, half of walnuts, and half of scallions in reserved bowl; add brown butter mixture and toss to coat. Transfer to a platter and scatter remaining walnuts and scallions over. Finely grate lemon zest on top; sprinkle with sea salt.