Cut the fish fillets into bite-sized pieces.
In a bowl, mix together the coconut milk, soy sauce, lime juice, brown sugar, minced garlic, and finely chopped onion. Add red chili if using.
Place the fish pieces in a shallow dish and pour the marinade over them. Cover and refrigerate for at least 30 minutes to allow the flavors to meld.
Heat vegetable oil in a large skillet or pan over medium heat. Remove the fish from the marinade and cook in the hot oil until golden brown and cooked through, about 4-5 minutes per side.
Garnish with fresh cilantro or parsley. Serve hot with rice or vegetables.
Enjoy your meal!