Cut off the head and belly of the mackerel, clean thoroughly, and let drain. Shred the bamboo shoots and boil them in water for about 5 minutes to remove any bitterness. Rinse several times with cold water and squeeze out excess water.

Heat 1 1/2 tbsp vegetable oil in a pan. When hot, add 1/2 tbsp minced shallot, 1/2 tbsp minced garlic, and 1/2 tbsp minced chili pepper. Sauté until fragrant, then add the bamboo shoots and stir-fry. Season with 1 tsp seasoning powder, mix well, and stir-fry for about 5 minutes until the bamboo shoots absorb the flavors. Transfer to a plate.

Heat another 1 1/2 tbsp vegetable oil in the pan. When hot, add 1/2 tbsp minced shallot and 1/2 tbsp minced garlic. Sauté until fragrant, then add 5 tbsp fish sauce and the mackerel. Cook until the fish absorbs the fish sauce and firms up. When the fish sauce has reduced, add the coconut water, 1 tbsp sugar, and 1/2 tsp caramelized coconut syrup. Bring to a gentle simmer.

Add all the stir-fried bamboo shoots to the pan with the fish. Season with 1/4 tsp salt and 1/4 tsp black pepper. Simmer on low heat until the liquid reduces to about two-thirds. Adjust seasoning to taste. Add chopped scallions and sliced chili pepper, gently stir, then turn off the heat. Transfer to a serving plate, garnish with more scallions and chili slices, and enjoy with hot rice.