In a small saucepan, heat and reduce the sugar until caramelized, 5 minutes. Add in the fish sauce, bring to a simmer, and cook until light caramel in color, 2 minutes. Then add in the fish stock and coconut water and bring to a simmer, 3 minutes.
In a separate pan, saute the onions, then add in the garlic and cook until fragrant and softened, 3 minutes. Add the cooked onions and garlic to the caramel sauce, then add in the chiles and spring onions. Place the fish in the pan, laying the thin tail of the fish on top of the spring onion, so it does not overcook. Cook until ready to serve, 5 minutes. Garnish with cilantro to serve.