Ask the fish seller to clean and debone the snakehead fish for you. If possible, have them scrub the fish with salt and rinse it thoroughly.

Bring about 6 1/3 cups (1.5 liters) of water to a boil with two smashed shallots and 1/2 teaspoon salt. Add the fish bones and simmer over low heat for about 10 minutes.

Cut the fish into bite-sized pieces. Marinate with 2 teaspoons fish sauce, 1/2 teaspoon salt, 1/2 teaspoon MSG (or seasoning powder), a pinch of pepper, and minced shallot. Mix well and let sit for about 15 minutes to absorb the flavors.

Heat about 1 tablespoon cooking oil in a pan and sauté shallots until fragrant. Add the marinated fish and cook until the fish firms up. For extra color and flavor, add 1 tablespoon annatto oil if desired.

Strain the fish bone broth to remove the bones and return the broth to the pot. Mix 1/2 teaspoon fermented shrimp paste with a little water, then add it to the broth. Add the cooked fish, bring to a boil, then add the bánh canh noodles. Let it simmer for a few more minutes, then adjust the seasoning to taste.

Ladle the noodle soup into bowls, top with green onions, cilantro, and fresh chili. Serve hot for best flavor.