Preheat oven to 425. For easy cleanup, line a baking sheet with aluminum foil.
Toss Brussels sprouts with 3 tablespoons oil. Spread out on prepared baking sheet. Lightly season with salt and pepper. Sauce will be salty so you don't need much.
Roast for 20 to 25 minutes, stirring halfway through.
In a pan, heat remaining oil over medium heat. Add ginger, garlic, and green onion. Cook for 1 minute.
Add red chile paste, chiles, soy sauce, 3 tablespoons sugar, rice vinegar, and 1/2 cup water to pan. In a small bowl, mix cornstarch with the 1 1/2 tablespoons water. Add to pan. Bring mixture to a simmer and cook until sauce thickens. Give sauce a taste and if it tastes salty and not very sweet, add the extra teaspoon of sugar.
Drizzle sauce over Brussels sprouts and sprinkle with peanuts.