Clean and pat dry the Mackerel (ikan kembung); make 2–3 shallow diagonal cuts on each side. Squeeze the juice of the Calamansi over the fish, then rub inside and out with a little Salt and a light coating of Cooking Oil.
In a blender or mortar, combine the Shallots, Garlic, Dried Red Chilies (soak briefly if very hard), Fresh Red Chilies, the white part of the Lemongrass, Galangal, Fresh Turmeric, Belacan (shrimp paste), Palm Sugar (gula melaka), a splash of Tamarind Paste and a little Water. Grind or blend to a coarse, spreadable paste.
Heat a tablespoon of cooking oil in a pan and fry the spice paste briefly until fragrant and slightly darkened, then add 1–2 tablespoons of Water to loosen if needed and simmer 1–2 minutes.
Spread about half of the cooled paste over the outside and into the cavity of the fish, reserving the rest for basting. Let the coated fish sit 15–30 minutes to absorb the flavors.
Warm the Banana Leaves over an open flame or in hot water until pliable. Lay a leaf flat, place the marinated fish on it, spoon a little of the reserved paste over the top, then wrap or fold the leaf around the fish and secure with toothpicks or skewers.
Grill the wrapped fish over medium heat (or cook on a hot cast-iron pan) for about 8–12 minutes per side depending on size, unwrapping for the last few minutes to char the skin and baste with the remaining paste and a little oil for a glossy finish.
Meanwhile make a simple sambal: heat a splash of oil, sauté sliced Red Onion until soft, add chopped Tomato and Bird's Eye Chilies and cook until the tomato breaks down. Stir in a pinch of Sugar, a little Tamarind Paste to taste and season with Salt.
When the fish is cooked through and nicely charred, transfer to a platter and serve hot with the tomato-chili sambal, extra calamansi halves for squeezing, and steamed rice or keropok on the side.
