Heat 2 Tbsp. of the oil in a large non-stick skillet or wok set over medium-high. Add the prawns and cook until just cooked through, about 2 minutes. Transfer to a plate and set aside. Add the remaining oil to the skillet or wok. When hot again, add the broccoli, onion and ginger and stir-fry 2 minutes. Add the black bean sauce, water and chili sauce and bring to a simmer. Stir-fry and simmer until the broccoli is tender/crisp, about 1 minute. Return the prawns to the pan and cook 1 minute more, or until heated through.