Cook potatoes and purple sweet potatoes, then mash until fluffy. Add whipping cream, butter, and a dash of salt and ground white pepper. Mix well. Keep warm and set aside.
Pat the eel meat dry and marinate lightly. Let meat air dry.
Stir seasoning mix well over hot water. Brush a layer onto the eel. Roast in a 180°C preheated oven for around 15 minutes. Continue to baste with seasoning mix periodically until the eel is cooked through. Plate the eel.
Stir sauce mix well over hot water. Drizzle over the eel. Serve with the mashed purple sweet potatoes.