Step 1To ensure clear broth, parboil the bones first by putting them in an 12-quart stockpot with just enough water to cover. Bring to a boil over high heat and boil until all the scum is released, 1 to 2 minutes. Drain in a colander and rinse until the scum comes off the bones. Scrub the pot, then add the rinsed bones and 6 quarts water. Bring to a boil over high heat, then add the rock sugar, onions and 5 teaspoons salt. Adjust the heat to maintain a simmer and simmer uncovered for 1½ hours, skimming any foam that rises to the surface.
Step 2While the broth simmers, prepare the leek-chile sauce and shrimp stir-fry.
Step 3When the broth is done, remove from the heat and let rest uncovered for 15 minutes to further concentrate its flavor and settle any remaining impurities.
Step 4While the broth rests, bring a large saucepan of water to a boil. Add the noodles and boil until just tender. The timing varies, so test for doneness by picking out a noodle, rinsing it under cold water and biting into it. It should be cooked through, but remain slightly firm. Drain in a fine colander, rinse under cold water until cool, and drain again. Divide the bean sprouts among 8 serving bowls, then top with the drained noodles.
Step 5Strain the broth through a fine-mesh sieve set over a 6-quart pot. Season with 3 tablespoons fish sauce. Taste and add more if desired. Bring the broth to a simmer over medium-high heat, then reduce the heat to maintain a steady simmer. Season the fish with salt, then add to the simmering broth and poach until the fish is just opaque throughout, 2 to 4 minutes. Using a slotted spoon or spider, scoop out the fish and divide among the serving bowls.
Step 6Divide the shrimp among the bowls as well, along with the cucumbers and rau ram. Add a spoonful of the leek-chile sauce to each bowl and place the remaining in a serving bowl for passing at the table. Sprinkle the scallion greens on top. Return the broth to a boil and divide it among the bowls. Crown with the fried shallots. Serve immediately with the leek-chile sauce, more fish sauce, lime wedges and mint sprigs.