Place catfish in a shallow pan, and sprinkle with oil and lemon grass or lemon zest. Set aside.
To make the sauce, heat oil in a small saucepan over medium heat. Add onion and garlic, and sauté for about 1 minute. Add hoisin sauce, water, vinegar and chili paste, if using. Reduce heat, and simmer for about 10 minutes or until a creamy consistency. If sauce is too thick, add more water. Set aside.
Prepare a grill, or preheat the broiler.
Place fillets on an oiled grill rack or broiler pan rack. Grill or broil fillets about 4 inches from the heat source for 2 or 3 minutes on each side, or until fish flakes easily when tested with a fork. Place fillets on serving plates, and spoon a little of the sauce on top. Sprinkle with green onions and peanuts, and serve.
Notes: Hoisin sauce and chili paste are available at Asian markets.