To make the Broth: In a large stockpot, combine all broth with the 8 cups (1920 ml) of water.
Cover the pot and bring to a rolling boil over high heat.
Once boiling, reduce the heat to medium-low and let it simmer, covered, for at least 1 hour to allow the flavors to meld together beautifully.
Carefully strain the broth through a fine-mesh sieve into a clean pot, discarding the solids. Return the fragrant broth to the stove and keep it warm over low heat.
To make the Pho: While the broth is simmering, cook the rice noodles according to package directions. To avoid mushy noodles, drain them just as they become tender and immediately rinse under cold water to stop the cooking process.
Divide the cooked noodles among six large soup bowls. Ladle the hot, steaming broth over the noodles.
Artfully arrange the sliced tofu, soybean sprouts, watercress, and sliced green onions on top.
Serve immediately with a side platter of fresh cilantro, basil leaves, and lime wedges for everyone to customize their own bowl.