In a large 8-quart pot, combine the water, diced onions, ginger slabs, shrimp paste, galangal, salt, pepper, turmeric, and halved chiles. Bring the liquid to a boil, then lower the heat and simmer for 15 minutes until the aromatics have infused the water and the shrimp paste has completely broken down.

Gently lower the whole fish into the broth. Simmer over medium-low heat for 12 to 15 minutes, carefully turning the fish halfway through if it is not submerged, until the flesh pulls away cleanly from the bone.

Using a spider or slotted spoon, lift the fish into a bowl. Once cool enough to handle, remove and discard the skin. Carefully flake the meat away from the bones and set it aside, keeping the skeleton as intact as possible. Return the bones, head, and tail to the pot.

Simmer the bones in the broth for another 15 minutes to deepen the flavor. Strain the entire broth through a fine-mesh strainer into a clean container, then rinse the pot and return the strained broth to it. Keep it at a very low simmer.

Heat the canola oil in a wok or large skillet over high heat until shimmering. Add the minced onions and cook until softened and translucent, about 4 minutes. Stir in the minced garlic, ginger, and galangal and fry for one minute until fragrant.

Add the reserved fish meat, turmeric, and salt to the wok. Mash the fish gently with a spatula into a coarse paste and cook for 2 minutes, picking out any small bones you might have missed.

Pour the fish paste into the simmering broth and bring to a brisk simmer for 5 minutes. Stir in the tamarind water. Taste the broth; it should be assertive and quite salty to compensate for the unseasoned noodles.

Boil a separate pot of water and cook the rice noodles until softened, about 5 to 6 minutes. Turn off the heat and let the noodles sit in the water for 3 minutes to finish, then drain and rinse under cool running water. Shake well to remove all excess moisture.

Divide the noodles among bowls. Stir the raw minced garlic and Thai chiles together in a small ramekin. Ladle the hot soup over the noodles and serve with the chile-garlic mixture, fried onions, and cilantro sprigs on the side.