Rinse the pork bones thoroughly and blanch them in boiling water. Place them in a pot with a pinch of salt, ground black pepper, and a few shallots. Simmer over low heat to keep the broth clear, skimming off any foam as needed.

Rinse the pounded crab and extract the liquid for cooking. For 7 oz (200 grams) of crab, use about 4 1/4 to 5 1/2 cups (1 to 1.3 liters) of water. Rinse gradually to get the richest crab stock. Add a pinch of salt for flavor, then simmer over medium-low heat until the crab floats to the top and the broth is clear. Remove the crab to a bowl and set aside. If you scoop up too much liquid, you can drain it to keep the crab mixture firm.

While the bone broth is simmering, heat oil or lard in a pan and fry the tofu pieces until golden. In the same oil, fry the sliced shallots (reserve some shallots for coloring the broth). Note: Fried shallots take time but are essential for flavor and texture. Remove the shallots from the pan as soon as they start to turn golden, as they will continue to cook and crisp up as they cool. For even cooking, slice the shallots evenly.

To color the broth: In a small pot, heat oil or lard and add the reserved shallots, frying until fragrant. Add 2 chopped tomatoes and cook until soft. Once the tomatoes are soft, add the crab mixture and 2 teaspoons fish sauce for flavor. Simmer over medium-low heat until the mixture is soft.

Prepare the broth: Pour some of the pork bone broth into the crab broth (after removing the crab mixture) to reach the desired amount. Add the tomato mixture to the broth and bring to a boil.

Place the tamarind paste in a small bowl and add 3 tablespoons (50 ml) hot water to extract the sour flavor. Gradually add this to the broth, tasting as you go until you reach about 60%-70% of your desired sourness. Adjust the remaining sourness with rice vinegar. Using tamarind will make the broth less clear than using only vinegar or pickled dracontomelon, but in my experience, 100% vinegar can be too sharp. Adjust the souring agent to your family's taste and local preferences.

Before serving, add the fried tofu and fresh tomato wedges to the simmering broth for a few minutes to absorb the flavors. Only add as much tofu as you plan to serve immediately for best texture; avoid adding all the tofu at once and letting it sit.

To assemble each bowl: Blanch a portion of rice vermicelli in boiling water, then drain. Place the noodles in a bowl and top with the prepared ingredients: crab mixture, chopped scallions, ladle in the broth with tomato wedges and tofu. Sprinkle fried shallots on top.

Serve bún riêu with fresh herbs and vegetables, and add fermented shrimp paste to taste. If you don't use noodles, the broth and toppings are still delicious with just the fresh herbs. Enjoy!