Squeeze the orange and mix with dark soy sauce, salt, and pepper to make a marinade. Place the catfish fillets in a container and pour the marinade over them. Refrigerate for 1 hour.
Heat oil in a pan over medium-high heat. Add the sesame seeds and toast for 2-3 minutes, stirring frequently, until fragrant. Transfer to a plate.
Increase the heat to high and sear the catfish fillets for 2 minutes on each side until a crust forms.
Pour the remaining marinade into the pan. Add the honey and stir gently. Reduce heat to medium and simmer for 3-4 minutes until the fish is cooked through (internal temperature 63 °C / 145 °F) and the sauce thickens slightly.
Sprinkle the toasted sesame seeds over the fish before serving.