Meanwhile, heat a drizzle of oil in a large pan over medium heat. Add onion and cook, stirring occasionally, until browned and softened, 8-10 minutes. (TIP: If onion begins to brown too quickly, add a splash of water.) Stir in vinegar and 1 tsp sugar (2 tsp for 4 servings). Continue cooking until caramelized and jammy, 2-3 minutes more. Season with salt and pepper. Turn off heat; transfer to a small bowl. Wash out pan.