Add cold water to the ground crab. Use only half the water for the first round of straining, about 3 cups (700 ml). Scoop small amounts of ground crab into a fine mesh strainer and press with a ladle to extract the liquid. Save the crab solids in a bowl for a second round of straining.

After the first round, add the remaining half of the cold water (about 3 cups/700 ml) and strain again. This second batch will be lighter. If you have a lot of crab but only need a small amount of broth, you can save this second batch of strained water to make soup with luffa and jute leaves or malabar spinach for another meal.

Once strained, bring the crab liquid to a boil. When the crab paste starts to float, lower the heat to medium-high (about 7 out of 10) so the paste doesn't dissolve. Gently stir to prevent sticking, and use a ladle to skim off the floating crab paste into a stainless steel strainer. Keep skimming until no more crab paste rises.

Let the crab paste drain in the strainer, then transfer it to a bowl. Once cooled, it will set into a nice block.

Store the crab paste in the fridge until ready to use. Mix in 200 grams (about 7 oz) ground pork and 1 duck egg, and combine well. If you use only crab paste, it can be a bit dry when served.

You can spoon the mixed paste directly into the soup pot, or for a nicer presentation, put it in a bowl, brush a chicken egg yolk on top, and steam until set. When serving, cut the crab paste into pieces to top each bowl of noodles.

Cut 2 tomatoes into wedges. Sauté shallots until fragrant, add 1 tablespoon annatto oil for color, then stir-fry the crab fat with a bit of fish sauce. Add the tomato wedges and 1 tablespoon fish sauce, simmer on low until the tomatoes are soft, then pour into the strained crab broth.

Cut the remaining 2 tomatoes into quarters and add them directly to the broth. Season with 3 tablespoons fish sauce, 1 tablespoon seasoning powder, 1 teaspoon rock sugar, and more salt if needed.

You can add a bit of rice vinegar for a light tangy flavor.

Cut the tofu into small cubes, fry until golden, then add to the pot so the tofu absorbs the broth and becomes tender and flavorful.

If you like crab cakes, spoon a few into the boiling broth; they’ll cook in about 1 minute.

If you have apple snails, boil them, remove the meat, and add to the noodles before ladling on the broth for bún riêu ốc.

Blanch the rice vermicelli noodles with boiling water to keep the bowls hot. Place pieces of crab paste on top of the noodles, ladle the broth over, and sprinkle with scallions and cilantro. Add crispy shallots if you like.

Serve with shredded lettuce, perilla, Vietnamese balm, and bean sprouts. Add a little fermented shrimp paste to your bowl if desired.