Prepare the Kamado BONO grill for indirect cooking by lighting the charcoal with a lighter, placing the cast iron grates, and waiting until the temperature rises to 180°C. Adjust the top and bottom air vents to maintain a stable temperature. Place the cast iron skillet on the grill.
Chop the onions and bell pepper, then sauté them in the skillet with some oil for about 10 minutes. Add water, season with sugar, and simmer for about 10 minutes. Remove from the skillet and blend into a smooth sauce.
Salsa:
Cut the vegetables into neat cubes, season with salt, pepper, vinegar, and soy sauce. Season the fish with salt and pepper, and grill over direct heat for 7 minutes.