The catfish type in this grill dish is the wild-caught walking catfish (Clarias batrachus, Vietnamese: cá trê), which is smaller and less fatty than other catfish types. Galangal x fermented rice is a common flavor pairing in North Vietnamese cuisine. Galangal (Vietnamese: riềng) is a rhizome, similar to ginger, but has a strong pungent and spicy taste. In fact, the common galagal in Vietnam is the lesser galangal (Alpinia officinarum), although the greater one (Alpinia galanga, Vietnamese: riềng nếp) might also do. Fermented rice (Vietnamese: mẻ) is a simple way to bring the acidity to food. So, you can expect from this dish is the umami flavor of the grilled fish, the spiciness of the galangal, and the sourness of the fermented. Several other ingredients are added, too, to balance the overall taste.
Rise the catfish and put in a plastic bag, add some salt, to remove the slimy substance on catfish skin. Add some kitchen ashes or baking soda to rub off the slime. Remove the fish organs including the slime gland near the upper fins, and the grill. Rinse again and let them dry.
Make some crimp cuts on the fish body, so seasonings will penetrate better. In a large bowl, add fish, fermented rice, rice wine, pounded galangal, lemon grass, ground black pepper, some cooking oil, sugar, fish sauce, salt, msg or granule seasoning. Rub and stuff the fish with the seasonings and marinate for 2 hours.
Turn on the oven 10 mins before grilling. Shake off the seasonings on the fish skin. Place the fish on a grill with a tray to collect dripping fluid. This dripping fluid will be used in a dipping sauce later. Grill until the fish are golden brown for about 50-60 mins, possibly longer if the fish are big.
Heat the oil in a saucepan, throw in some sliced lemongrass to sauté, and scoop them out for later use. Add the leftover fish marinade and the fish dripping fluid to the saucepan, and then add some water, and stir. Taste and re-season with some salt, fish sauce or sugar to suit your taste buds. The sauce should be yellow, slightly sour, slightly sweet and aromatic.
Place the fish on a serving plate with spring onions, dill leaves, salad leaves, sliced cucumber and the sautéed lemongrass slices.