Recipe developer and fashion designer Peter Som didn’t grow up with spicy food. Cantonese dishes don’t have the heat of Sichuan or Hunan cooking, and it was only many years later, after he moved to New York, that Som came to appreciate the wide range of Asian cuisines that balance heat and sweet. The combination of Korean gochujang, maple syrup, and brown butter creates that ideal union in this side dish, a staple at his Thanksgiving table. If you can’t find gochujang try using 2–3 Tbsp. Sriracha instead.
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.
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.
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.
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.