Step 1
In a small bowl, stir together the fish curry masala and 2 tablespoons of water.
Step 2
To a large skillet over medium-high heat, add the oil. When it is hot but not smoking, add the fennel, fenugreek, black mustard seeds, and curry leaves and cook, stirring occasionally, until the leaves start to sputter, about 1 minute. Add the chiles and shallots and cook, stirring occasionally, until the shallots are soft and translucent, about 5 minutes. Add the ginger-garlic paste and tomato and cook, stirring occasionally, until the tomato is soft, 2–3 minutes. Add the fish curry masala paste, season to taste with salt, and continue cooking, stirring continuously, until fragrant, about 3 minutes more.
Step 3
Add the coconut milk and kudampuli (along with its soaking liquid), turn the heat to high, and boil until slightly thickened, about 5 minutes. Add the fish, cook for 3 minutes, then cover, turn the heat to low, and simmer until the fish is cooked through, about 5 minutes more.