Preheat oven to 450°. Slice broccoli stems on a diagonal ¼” thick. Transfer to a rimmed baking sheet, toss with oil, and season with kosher salt and pepper. Gather up loose pieces of left-behind florets and finely chop. Roast stems until browned around edges, 15–20 minutes. Add vinegar; toss to coat.

Meanwhile, heat a dry medium skillet, preferably cast iron, over medium-high. Add florets. Season with kosher salt and pepper and cook, stirring often, until bright green and lightly charred in spots, about 5 minutes. Reduce heat to low; add peanuts and sugar. Cook, stirring, until nuts are golden brown. Stir in 2 Tbsp. nutritional yeast; season again.

Serve broccoli stems and florets topped with scallions, sea salt, and more yeas